diff --git a/redo-ifchange.py b/redo-ifchange.py
index a70fc03..99ffad3 100755
--- a/redo-ifchange.py
+++ b/redo-ifchange.py
@@ -5,9 +5,14 @@ from helpers import debug, err, mkdirp, unlink
 
 
 def _dirty_deps(t, depth, fromdir):
-    if state.ismarked(t, fromdir):
-        return False  # has already been checked during this session
     debug('%s?%s\n' % (depth, t))
+    if state.isbuilt(t, fromdir):
+        debug('%s-- DIRTY (built)\n' % depth)
+        return True  # has already been built during this session
+    if state.ismarked(t, fromdir):
+        debug('%s-- CLEAN (marked)\n' % depth)
+        return False  # has already been checked during this session
+    
     stamptime = state.stamped(t, fromdir)
     if stamptime == None:
         debug('%s-- DIRTY (no stamp)\n' % depth)
@@ -29,7 +34,7 @@ def _dirty_deps(t, depth, fromdir):
                 return True
         elif mode == 'm':
             if dirty_deps(name, depth + '  ', fromdir=vars.BASE):
-                #debug('%s-- DIRTY (sub)\n' % depth)
+                debug('%s-- DIRTY (sub)\n' % depth)
                 return True
     state.mark(t, fromdir)
     return False
@@ -42,8 +47,12 @@ def dirty_deps(t, depth, fromdir=None):
     return False
 
 
+def should_build(t):
+    return not state.isbuilt(t) and dirty_deps(t, depth = '')
+
+
 def maybe_build(t):
-    if dirty_deps(t, depth = ''):
+    if should_build(t):
         builder.build(t)
 
 
@@ -56,7 +65,7 @@ try:
     want_build = []
     for t in sys.argv[1:]:
         state.add_dep(vars.TARGET, 'm', t)
-        if dirty_deps(t, depth = ''):
+        if should_build(t):
             want_build.append(t)
 
     rv = builder.main(want_build, maybe_build)
diff --git a/state.py b/state.py
index b8a7b33..4c26769 100644
--- a/state.py
+++ b/state.py
@@ -13,6 +13,8 @@ def init():
         os.unlink(f)
     for f in glob.glob('%s/.redo/mark^*' % vars.BASE):
         os.unlink(f)
+    for f in glob.glob('%s/.redo/built^*' % vars.BASE):
+        os.unlink(f)
 
 
 def _sname(typ, t, fromdir=None):
@@ -46,6 +48,7 @@ def _stampname(t, fromdir=None):
     
 
 def stamp(t):
+    built(t)
     mark(t)
     stampfile = _stampname(t)
     newstampfile = _sname('stamp' + str(os.getpid()), t)
@@ -78,6 +81,25 @@ def stamped(t, fromdir=None):
     return stamptime
 
 
+def built(t, fromdir=None):
+    try:
+        open(_sname('built', t, fromdir), 'w').close()
+    except IOError, e:
+        if e.errno == errno.ENOENT:
+            pass  # may happen if someone deletes our .redo dir
+        else:
+            raise
+
+
+_builts = {}
+def isbuilt(t, fromdir=None):
+    if _builts.get((t,fromdir)):
+        return True
+    if os.path.exists(_sname('built', t, fromdir)):
+        _builts[(t,fromdir)] = True
+        return True
+
+
 def mark(t, fromdir=None):
     try:
         open(_sname('mark', t, fromdir), 'w').close()