diff --git a/irc/caps/set.go b/irc/caps/set.go
index 867617b3..b348cbf8 100644
--- a/irc/caps/set.go
+++ b/irc/caps/set.go
@@ -59,6 +59,11 @@ func (s *Set) Union(other *Set) {
 	utils.BitsetUnion(s[:], other[:])
 }
 
+// Subtract removes all the capabilities of another set from this set.
+func (s *Set) Subtract(other *Set) {
+	utils.BitsetSubtract(s[:], other[:])
+}
+
 // Empty returns whether the set is empty.
 func (s *Set) Empty() bool {
 	return utils.BitsetEmpty(s[:])
diff --git a/irc/caps/set_test.go b/irc/caps/set_test.go
index 019ca76b..c32e25e6 100644
--- a/irc/caps/set_test.go
+++ b/irc/caps/set_test.go
@@ -60,6 +60,17 @@ func TestSets(t *testing.T) {
 	}
 }
 
+func TestSubtract(t *testing.T) {
+	s1 := NewSet(AccountTag, EchoMessage, UserhostInNames, ServerTime)
+
+	toRemove := NewSet(UserhostInNames, EchoMessage)
+	s1.Subtract(toRemove)
+
+	if !reflect.DeepEqual(s1, NewSet(AccountTag, ServerTime)) {
+		t.Errorf("subtract doesn't work")
+	}
+}
+
 func BenchmarkSetReads(b *testing.B) {
 	set := NewSet(UserhostInNames, EchoMessage)
 	b.ResetTimer()
diff --git a/irc/handlers.go b/irc/handlers.go
index b8075296..adfe3b0e 100644
--- a/irc/handlers.go
+++ b/irc/handlers.go
@@ -450,19 +450,27 @@ func awayHandler(server *Server, client *Client, msg ircmsg.IrcMessage, rb *Resp
 // CAP <subcmd> [<caps>]
 func capHandler(server *Server, client *Client, msg ircmsg.IrcMessage, rb *ResponseBuffer) bool {
 	subCommand := strings.ToUpper(msg.Params[0])
-	capabilities := caps.NewSet()
+	toAdd := caps.NewSet()
+	toRemove := caps.NewSet()
 	var capString string
 
-	var badCaps []string
+	badCaps := false
 	if len(msg.Params) > 1 {
 		capString = msg.Params[1]
 		strs := strings.Fields(capString)
 		for _, str := range strs {
+			remove := false
+			if str[0] == '-' {
+				str = str[1:]
+				remove = true
+			}
 			capab, err := caps.NameToCapability(str)
-			if err != nil || !SupportedCapabilities.Has(capab) {
-				badCaps = append(badCaps, str)
+			if err != nil || (!remove && !SupportedCapabilities.Has(capab)) {
+				badCaps = true
+			} else if !remove {
+				toAdd.Enable(capab)
 			} else {
-				capabilities.Enable(capab)
+				toRemove.Enable(capab)
 			}
 		}
 	}
@@ -490,16 +498,17 @@ func capHandler(server *Server, client *Client, msg ircmsg.IrcMessage, rb *Respo
 		}
 
 		// make sure all capabilities actually exist
-		if len(badCaps) > 0 {
+		if badCaps {
 			rb.Add(nil, server.name, "CAP", client.nick, "NAK", capString)
 			return false
 		}
-		client.capabilities.Union(capabilities)
+		client.capabilities.Union(toAdd)
+		client.capabilities.Subtract(toRemove)
 		rb.Add(nil, server.name, "CAP", client.nick, "ACK", capString)
 
 		// if this is the first time the client is requesting a resume token,
 		// send it to them
-		if capabilities.Has(caps.Resume) {
+		if toAdd.Has(caps.Resume) {
 			token, err := client.generateResumeToken()
 			if err == nil {
 				rb.Add(nil, server.name, "RESUME", "TOKEN", token)
diff --git a/irc/utils/bitset.go b/irc/utils/bitset.go
index 514d2fac..9e0014a8 100644
--- a/irc/utils/bitset.go
+++ b/irc/utils/bitset.go
@@ -89,3 +89,19 @@ func BitsetCopy(set []uint64, other []uint64) {
 		atomic.StoreUint64(&set[i], data)
 	}
 }
+
+// BitsetSubtract modifies `set` to subtract the contents of `other`.
+// Similar caveats about race conditions as with `BitsetUnion` apply.
+func BitsetSubtract(set []uint64, other []uint64) {
+	for i := 0; i < len(set); i++ {
+		for {
+			ourAddr := &set[i]
+			ourBlock := atomic.LoadUint64(ourAddr)
+			otherBlock := atomic.LoadUint64(&other[i])
+			newBlock := ourBlock & (^otherBlock)
+			if atomic.CompareAndSwapUint64(ourAddr, ourBlock, newBlock) {
+				break
+			}
+		}
+	}
+}
diff --git a/irc/utils/bitset_test.go b/irc/utils/bitset_test.go
index 282f1c6f..a34a0097 100644
--- a/irc/utils/bitset_test.go
+++ b/irc/utils/bitset_test.go
@@ -73,8 +73,13 @@ func TestSets(t *testing.T) {
 	BitsetCopy(t3s, t1s)
 	for i = 0; i < 128; i++ {
 		expected := (i != 72)
-		if BitsetGet(t1s, i) != expected {
+		if BitsetGet(t3s, i) != expected {
 			t.Error("all bits should be set except 72")
 		}
 	}
+
+	BitsetSubtract(t3s, t2s)
+	if !BitsetGet(t3s, 0) || BitsetGet(t3s, 72) || !BitsetGet(t3s, 74) || BitsetGet(t3s, 71) {
+		t.Error("subtract doesn't work")
+	}
 }