diff --git a/inhibit.go b/inhibit.go
index 1669d11a..7f5a9cf6 100644
--- a/inhibit.go
+++ b/inhibit.go
@@ -15,6 +15,7 @@ package main
 
 import (
 	"sync"
+	"time"
 
 	"github.com/prometheus/common/log"
 	"github.com/prometheus/common/model"
@@ -31,7 +32,8 @@ type Inhibitor struct {
 	rules  []*InhibitRule
 	marker types.Marker
 
-	mtx sync.RWMutex
+	mtx   sync.RWMutex
+	stopc chan struct{}
 }
 
 // NewInhibitor returns a new Inhibitor.
@@ -41,41 +43,87 @@ func NewInhibitor(ap provider.Alerts, rs []*config.InhibitRule, mk types.Marker)
 		marker: mk,
 	}
 	for _, cr := range rs {
-		ih.rules = append(ih.rules, NewInhibitRule(cr))
+		r := NewInhibitRule(cr)
+		ih.rules = append(ih.rules, r)
 	}
 	return ih
 }
 
-// Mutes returns true iff the given label set is muted.
-func (ih *Inhibitor) Mutes(lset model.LabelSet) bool {
-	alerts := ih.alerts.GetPending()
-	defer alerts.Close()
-
-	// TODO(fabxc): improve erroring for iterators so it does not
-	// go silenced here.
-
-	for alert := range alerts.Next() {
-		if err := alerts.Err(); err != nil {
-			log.Errorf("Error iterating alerts: %s", err)
-			continue
+func (ih *Inhibitor) runGC() {
+	for {
+		select {
+		case <-time.After(15 * time.Minute):
+			for _, r := range ih.rules {
+				r.gc()
+			}
+		case <-ih.stopc:
+			return
 		}
-		if alert.Resolved() {
-			continue
-		}
-		for _, rule := range ih.rules {
-			if rule.Mutes(alert.Labels, lset) {
-				ih.marker.SetInhibited(lset.Fingerprint(), true)
-				return true
+	}
+}
+
+// Run the Inihibitor's background processing.
+func (ih *Inhibitor) Run() {
+	ih.mtx.Lock()
+	ih.stopc = make(chan struct{})
+	ih.mtx.Unlock()
+
+	go ih.runGC()
+
+	it := ih.alerts.Subscribe()
+	defer it.Close()
+
+	for {
+		select {
+		case <-ih.stopc:
+			return
+		case a := <-it.Next():
+			if err := it.Err(); err != nil {
+				log.Errorf("Error iterating alerts: %s", err)
+				continue
+			}
+			if a.Resolved() {
+				// As alerts can also time out without an update, we never
+				// handle new resolved alerts but invalidate the cache on read.
+				continue
+			}
+			// Populate the inhibition rules' cache.
+			for _, r := range ih.rules {
+				if r.SourceMatchers.Match(a.Labels) {
+					r.set(a)
+				}
 			}
 		}
 	}
-	if err := alerts.Err(); err != nil {
-		log.Errorf("Error after iterating alerts: %s", err)
+}
+
+// Stop the Inhibitor's background processing.
+func (ih *Inhibitor) Stop() {
+	if ih == nil {
+		return
 	}
+	ih.mtx.Lock()
+	defer ih.mtx.Unlock()
 
-	ih.marker.SetInhibited(lset.Fingerprint(), false)
+	if ih.stopc != nil {
+		close(ih.stopc)
+		ih.stopc = nil
+	}
+}
 
+// Mutes returns true iff the given label set is muted.
+func (ih *Inhibitor) Mutes(lset model.LabelSet) bool {
+	fp := lset.Fingerprint()
+
+	for _, r := range ih.rules {
+		if r.TargetMatchers.Match(lset) && r.hasEqual(lset) {
+			ih.marker.SetInhibited(fp, true)
+			return true
+		}
+	}
+	ih.marker.SetInhibited(fp, false)
 	return false
+
 }
 
 // An InhibitRule specifies that a class of (source) alerts should inhibit
@@ -93,6 +141,10 @@ type InhibitRule struct {
 	// A set of label names whose label values need to be identical in source and
 	// target alerts in order for the inhibition to take effect.
 	Equal map[model.LabelName]struct{}
+
+	mtx sync.RWMutex
+	// Cache of alerts matching source labels.
+	scache map[model.Fingerprint]*types.Alert
 }
 
 // NewInhibitRule returns a new InihibtRule based on a configuration definition.
@@ -125,20 +177,48 @@ func NewInhibitRule(cr *config.InhibitRule) *InhibitRule {
 		SourceMatchers: sourcem,
 		TargetMatchers: targetm,
 		Equal:          equal,
+		scache:         map[model.Fingerprint]*types.Alert{},
 	}
 }
 
-// Mutes returns true iff the Inhibition rule applies for the given
-// source and target label set.
-func (r *InhibitRule) Mutes(source, target model.LabelSet) bool {
-	if !r.TargetMatchers.Match(target) || !r.SourceMatchers.Match(source) {
-		return false
+// set the alert in the source cache.
+func (r *InhibitRule) set(a *types.Alert) {
+	r.mtx.Lock()
+	r.mtx.Unlock()
+
+	r.scache[a.Fingerprint()] = a
+}
+
+// hasEqual checks whether the source cache contains alerts matching
+// the equal labels for the given label set.
+func (r *InhibitRule) hasEqual(lset model.LabelSet) bool {
+	r.mtx.RLock()
+	defer r.mtx.RUnlock()
+
+Outer:
+	for _, a := range r.scache {
+		// The cache might be stale and contain resolved alerts.
+		if a.Resolved() {
+			continue
+		}
+		for n := range r.Equal {
+			if a.Labels[n] != lset[n] {
+				continue Outer
+			}
+		}
+		return true
 	}
-	for ln := range r.Equal {
-		if source[ln] != target[ln] {
-			return false
+	return false
+}
+
+// gc clears out resolved alerts from the source cache.
+func (r *InhibitRule) gc() {
+	r.mtx.Lock()
+	defer r.mtx.Unlock()
+
+	for fp, a := range r.scache {
+		if a.Resolved() {
+			delete(r.scache, fp)
 		}
 	}
-
-	return true
 }
diff --git a/inhibit_test.go b/inhibit_test.go
new file mode 100644
index 00000000..f212471d
--- /dev/null
+++ b/inhibit_test.go
@@ -0,0 +1,168 @@
+// Copyright 2016 Prometheus Team
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"reflect"
+	"testing"
+	"time"
+
+	"github.com/kylelemons/godebug/pretty"
+	"github.com/prometheus/alertmanager/types"
+	"github.com/prometheus/common/model"
+)
+
+func TestInhibitRuleHasEqual(t *testing.T) {
+	now := time.Now()
+	cases := []struct {
+		initial map[model.Fingerprint]*types.Alert
+		equal   model.LabelNames
+		input   model.LabelSet
+		result  bool
+	}{
+		{
+			// No source alerts at all.
+			initial: map[model.Fingerprint]*types.Alert{},
+			input:   model.LabelSet{"a": "b"},
+			result:  false,
+		},
+		{
+			// No equal labels, any source alerts satisfies the requirement.
+			initial: map[model.Fingerprint]*types.Alert{1: &types.Alert{}},
+			input:   model.LabelSet{"a": "b"},
+			result:  true,
+		},
+		{
+			// Matching but already resolved.
+			initial: map[model.Fingerprint]*types.Alert{
+				1: &types.Alert{
+					Alert: model.Alert{
+						Labels:   model.LabelSet{"a": "b", "b": "f"},
+						StartsAt: now.Add(-time.Minute),
+						EndsAt:   now.Add(-time.Second),
+					},
+				},
+				2: &types.Alert{
+					Alert: model.Alert{
+						Labels:   model.LabelSet{"a": "b", "b": "c"},
+						StartsAt: now.Add(-time.Minute),
+						EndsAt:   now.Add(-time.Second),
+					},
+				},
+			},
+			equal:  model.LabelNames{"a", "b"},
+			input:  model.LabelSet{"a": "b", "b": "c"},
+			result: false,
+		},
+		{
+			// Matching but already resolved.
+			initial: map[model.Fingerprint]*types.Alert{
+				1: &types.Alert{
+					Alert: model.Alert{
+						Labels:   model.LabelSet{"a": "b", "c": "d"},
+						StartsAt: now.Add(-time.Minute),
+						EndsAt:   now.Add(-time.Second),
+					},
+				},
+				2: &types.Alert{
+					Alert: model.Alert{
+						Labels:   model.LabelSet{"a": "b", "c": "f"},
+						StartsAt: now.Add(-time.Minute),
+						EndsAt:   now.Add(-time.Second),
+					},
+				},
+			},
+			equal:  model.LabelNames{"a"},
+			input:  model.LabelSet{"a": "b"},
+			result: false,
+		},
+		{
+			// Equal label does not match.
+			initial: map[model.Fingerprint]*types.Alert{
+				1: &types.Alert{
+					Alert: model.Alert{
+						Labels:   model.LabelSet{"a": "c", "c": "d"},
+						StartsAt: now.Add(-time.Minute),
+						EndsAt:   now.Add(-time.Second),
+					},
+				},
+				2: &types.Alert{
+					Alert: model.Alert{
+						Labels:   model.LabelSet{"a": "c", "c": "f"},
+						StartsAt: now.Add(-time.Minute),
+						EndsAt:   now.Add(-time.Second),
+					},
+				},
+			},
+			equal:  model.LabelNames{"a"},
+			input:  model.LabelSet{"a": "b"},
+			result: false,
+		},
+	}
+
+	for _, c := range cases {
+		r := &InhibitRule{
+			Equal:  map[model.LabelName]struct{}{},
+			scache: map[model.Fingerprint]*types.Alert{},
+		}
+		for _, ln := range c.equal {
+			r.Equal[ln] = struct{}{}
+		}
+		for k, v := range c.initial {
+			r.scache[k] = v
+		}
+
+		if have := r.hasEqual(c.input); have != c.result {
+			t.Errorf("Unexpected result %q, expected %q", have, c.result)
+		}
+		if !reflect.DeepEqual(r.scache, c.initial) {
+			t.Errorf("Cache state unexpectedly changed")
+			t.Errorf(pretty.Compare(r.scache, c.initial))
+		}
+	}
+}
+
+func TestInhibitRuleGC(t *testing.T) {
+	// TODO(fabxc): add now() injection function to Resolved() to remove
+	// dependency on machine time in this test.
+	now := time.Now()
+	newAlert := func(start, end time.Duration) *types.Alert {
+		return &types.Alert{
+			Alert: model.Alert{
+				Labels:   model.LabelSet{"a": "b"},
+				StartsAt: now.Add(start * time.Minute),
+				EndsAt:   now.Add(end * time.Minute),
+			},
+		}
+	}
+
+	before := map[model.Fingerprint]*types.Alert{
+		0: newAlert(-10, -5),
+		1: newAlert(10, 20),
+		2: newAlert(-10, 10),
+		3: newAlert(-10, -1),
+	}
+	after := map[model.Fingerprint]*types.Alert{
+		1: newAlert(10, 20),
+		2: newAlert(-10, 10),
+	}
+
+	r := &InhibitRule{scache: before}
+	r.gc()
+
+	if !reflect.DeepEqual(r.scache, after) {
+		t.Errorf("Unexpected cache state after GC")
+		t.Errorf(pretty.Compare(r.scache, after))
+	}
+}
diff --git a/main.go b/main.go
index 52141e49..2ac77b3d 100644
--- a/main.go
+++ b/main.go
@@ -171,12 +171,14 @@ func main() {
 		}
 		tmpl.ExternalURL = amURL
 
+		inhibitor.Stop()
 		disp.Stop()
 
 		inhibitor = NewInhibitor(alerts, conf.InhibitRules, marker)
 		disp = NewDispatcher(alerts, NewRoute(conf.Route, nil), build(conf.Receivers), marker)
 
 		go disp.Run()
+		go inhibitor.Run()
 
 		return nil
 	}