diff --git a/Annex.hs b/Annex.hs
index e057bb9d27..9beded53f5 100644
--- a/Annex.hs
+++ b/Annex.hs
@@ -46,6 +46,7 @@ import Git.CheckAttr
 import Git.CheckIgnore
 import Git.SharedRepository
 import qualified Git.Queue
+import Types.Key
 import Types.Backend
 import Types.GitConfig
 import qualified Types.Remote
@@ -112,6 +113,7 @@ data AnnexState = AnnexState
 	, inodeschanged :: Maybe Bool
 	, useragent :: Maybe String
 	, errcounter :: Integer
+	, unusedkeys :: Maybe (S.Set Key)
 	}
 
 newState :: GitConfig -> Git.Repo -> AnnexState
@@ -148,6 +150,7 @@ newState c r = AnnexState
 	, inodeschanged = Nothing
 	, useragent = Nothing
 	, errcounter = 0
+	, unusedkeys = Nothing
 	}
 
 {- Makes an Annex state object for the specified git repo.
diff --git a/Annex/FileMatcher.hs b/Annex/FileMatcher.hs
index b26a0d7ace..c144920cfb 100644
--- a/Annex/FileMatcher.hs
+++ b/Annex/FileMatcher.hs
@@ -65,6 +65,7 @@ parseToken checkpresent checkpreferreddir groupmap t
 	| t `elem` tokens = Right $ token t
 	| t == "present" = use checkpresent
 	| t == "inpreferreddir" = use checkpreferreddir
+	| t == "unused" = Right (Operation limitUnused)
 	| otherwise = maybe (Left $ "near " ++ show t) use $ M.lookup k $
 		M.fromList
 			[ ("include", limitInclude)
diff --git a/Limit.hs b/Limit.hs
index 6ce4443255..8250544a45 100644
--- a/Limit.hs
+++ b/Limit.hs
@@ -30,6 +30,7 @@ import Types.Group
 import Types.FileMatcher
 import Types.Limit
 import Logs.Group
+import Logs.Unused
 import Utility.HumanTime
 import Utility.DataUnits
 
@@ -199,6 +200,15 @@ limitLackingCopies approx want = case readish want of
 		return $ numcopies - length us >= needed
 	approxNumCopies = fromMaybe defaultNumCopies <$> getGlobalNumCopies
 
+{- Match keys that are unused.
+ - 
+ - This has a nice optimisation: When a file exists,
+ - its key is obviously not unused.
+ -}
+limitUnused :: MatchFiles
+limitUnused _ (MatchingFile _) = return False
+limitUnused _ (MatchingKey k) = S.member k <$> unusedKeys
+
 {- Adds a limit to skip files not believed to be present in all
  - repositories in the specified group. -}
 addInAllGroup :: String -> Annex ()
diff --git a/Logs/Unused.hs b/Logs/Unused.hs
index daeeb93aba..74f46b85ed 100644
--- a/Logs/Unused.hs
+++ b/Logs/Unused.hs
@@ -21,14 +21,17 @@ module Logs.Unused (
 	readUnusedLog,
 	readUnusedMap,
 	unusedKeys,
+	unusedKeys'
 ) where
 
 import qualified Data.Map as M
+import qualified Data.Set as S
 import Data.Time.Clock.POSIX
 import Data.Time
 import System.Locale
 
 import Common.Annex
+import qualified Annex
 import Types.Key
 import Utility.Tmp
 
@@ -84,5 +87,16 @@ readUnusedLog prefix = do
 readUnusedMap :: FilePath -> Annex UnusedMap
 readUnusedMap = log2map <$$> readUnusedLog
 
-unusedKeys :: Annex [Key]
-unusedKeys = M.keys <$> readUnusedLog ""
+{- Set of unused keys. This is cached for speed. -}
+unusedKeys :: Annex (S.Set Key)
+unusedKeys = maybe (setUnusedKeys =<< unusedKeys') return
+	=<< Annex.getState Annex.unusedkeys
+
+unusedKeys' :: Annex [Key]
+unusedKeys' = M.keys <$> readUnusedLog ""
+
+setUnusedKeys :: [Key] -> Annex (S.Set Key)
+setUnusedKeys ks = do
+	let v = S.fromList ks
+	Annex.changeState $ \s -> s { Annex.unusedkeys = Just v }
+	return v
diff --git a/Seek.hs b/Seek.hs
index 57bedfc848..3242dfb338 100644
--- a/Seek.hs
+++ b/Seek.hs
@@ -133,9 +133,9 @@ withKeyOptions keyop fallbackop params = do
 	auto <- Annex.getState Annex.auto
 	case    (allkeys || bare , unused, auto ) of
 		(True    , False , False) -> go loggedKeys
-		(False   , True  , False) -> go unusedKeys
+		(False   , True  , False) -> go unusedKeys'
 		(True    , True  , _    )
-			| bare && not allkeys -> go unusedKeys
+			| bare && not allkeys -> go unusedKeys'
 			| otherwise -> error "Cannot use --all with --unused."
 		(False   , False , _    ) -> fallbackop params
 		(_       , _     , True )
diff --git a/debian/changelog b/debian/changelog
index aba8a5d3fb..7c2a0d3b3c 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -14,7 +14,8 @@ git-annex (5.20140118) UNRELEASED; urgency=medium
     command is used to set the global number of copies, any annex.numcopies
     git configs will be ignored.
   * assistant: Make the prefs page set the global numcopies.
-  * Add lackingcopies and approxlackingcopies preferred content expressions.
+  * Add lackingcopies, approxlackingcopies, and unused to
+    preferred content expressions.
   * Client, transfer, incremental backup, and archive repositories
     now want to get content that does not yet have enough copies.
   * repair: Check git version at run time.
diff --git a/doc/todo/Limit_file_revision_history.mdwn b/doc/todo/Limit_file_revision_history.mdwn
index 9cdfe5e9b1..c00b555b18 100644
--- a/doc/todo/Limit_file_revision_history.mdwn
+++ b/doc/todo/Limit_file_revision_history.mdwn
@@ -39,17 +39,16 @@ Finally, how to specify a feature request for git-annex?
 > So, let's spec out a design.
 > 
 > * Add preferred content terminal to configure whether a repository wants
->   to hang on to unused content.
->   Something like "unused=true" I suppose, because not having a parameter
->   would complicate preferred content parsing, and I cannot think
->   of a useful parameter. (It cannot be a timestamp, because there's
->   no way repos can agree on about when a key became unused.)
+>   to hang on to unused content. Simply `unused`.
+>   (It cannot include a timestamp, because there's
+>   no way repos can agree on about when a key became unused.) **done**
 > * In order to quickly match that terminal, the Annex monad will need
->   to keep a Set of unused Keys. This should only be loaded on demand.  
+>   to keep a Set of unused Keys. This should only be loaded on demand.
+>   **done**  
 >   NB: There is some potential for a great many unused Keys to cause
 >   memory usage to balloon.
 > * Client repositories will end their preferred content with
->   `and unused=false`. Transfer repositories too, because typically
+>   `and (not unused)`. Transfer repositories too, because typically
 >   only client repos connect to them, and so otherwise unused files
 >   would build up there. Backup repos would want unused files. I
 >   think that archive repos would too.
@@ -90,7 +89,7 @@ Finally, how to specify a feature request for git-annex?
 > client directly edits it, or deletes it, it loses the old version,
 > but the other client will still be storing that old version.
 > 
-> ## Stability analysis for unused= in preferred content expressions
+> ## Stability analysis for unused in preferred content expressions
 > 
 > This is tricky, because two repos that are otherwise entirely
 > in sync may have differing opinons about whether a key is unused,