diff --git a/redo-ifchange.py b/redo-ifchange.py
index a9951b7..a70fc03 100755
--- a/redo-ifchange.py
+++ b/redo-ifchange.py
@@ -5,6 +5,8 @@ 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))
     stamptime = state.stamped(t, fromdir)
     if stamptime == None:
@@ -29,6 +31,7 @@ def _dirty_deps(t, depth, fromdir):
             if dirty_deps(name, depth + '  ', fromdir=vars.BASE):
                 #debug('%s-- DIRTY (sub)\n' % depth)
                 return True
+    state.mark(t, fromdir)
     return False
 
 
diff --git a/redo.py b/redo.py
index b371d24..263e2fb 100755
--- a/redo.py
+++ b/redo.py
@@ -50,6 +50,8 @@ if is_root:
     mkdirp('%s/.redo' % base)
     for f in glob.glob('%s/.redo/lock*' % base):
         os.unlink(f)
+    for f in glob.glob('%s/.redo/mark^*' % base):
+        os.unlink(f)
 
 
 if not vars.DEPTH:
diff --git a/state.py b/state.py
index 2857c36..3f8591d 100644
--- a/state.py
+++ b/state.py
@@ -34,6 +34,7 @@ def _stampname(t, fromdir=None):
     
 
 def stamp(t):
+    mark(t)
     stampfile = _stampname(t)
     newstampfile = _sname('stamp' + str(os.getpid()), t)
     depfile = _sname('dep', t)
@@ -65,6 +66,25 @@ def stamped(t, fromdir=None):
     return stamptime
 
 
+def mark(t, fromdir=None):
+    try:
+        open(_sname('mark', t, fromdir), 'w').close()
+    except IOError, e:
+        if e.errno == errno.ENOENT:
+            pass  # may happen if someone deletes our .redo dir
+        else:
+            raise
+
+
+_marks = {}
+def ismarked(t, fromdir=None):
+    if _marks.get((t,fromdir)):
+        return True
+    if os.path.exists(_sname('mark', t, fromdir)):
+        _marks[(t,fromdir)] = True
+        return True
+
+
 def is_generated(t):
     return os.path.exists(_sname('gen', t))