diff --git a/hgext/rebase.py b/hgext/rebase.py
--- a/hgext/rebase.py
+++ b/hgext/rebase.py
@@ -599,8 +599,7 @@
         if newwd < 0:
             # original directory is a parent of rebase set root or ignored
             newwd = self.originalwd
-        if (newwd not in [c.rev() for c in repo[None].parents()] and
-                not self.inmemory):
+        if newwd not in [c.rev() for c in repo[None].parents()]:
             ui.note(_("update back to initial working directory parent\n"))
             hg.updaterepo(repo, newwd, False)
 
@@ -957,20 +956,10 @@
                 ui.status(_('nothing to rebase from %s to %s\n') %
                           ('+'.join(bytes(repo[r]) for r in base), dest))
             return None
-    # If rebasing the working copy parent, force in-memory merge to be off.
-    #
-    # This is because the extra work of checking out the newly rebased commit
-    # outweights the benefits of rebasing in-memory, and executing an extra
-    # update command adds a bit of overhead, so better to just do it on disk. In
-    # all other cases leave it on.
-    #
-    # Note that there are cases where this isn't true -- e.g., rebasing large
-    # stacks that include the WCP. However, I'm not yet sure where the cutoff
-    # is.
+
     rebasingwcp = repo['.'].rev() in rebaseset
     ui.log("rebase", "", rebase_rebasing_wcp=rebasingwcp)
     if rbsrt.inmemory and rebasingwcp:
-        rbsrt.inmemory = False
         # Check these since we did not before.
         cmdutil.checkunfinished(repo)
         cmdutil.bailifchanged(repo)
diff --git a/tests/test-rebase-inmemory.t b/tests/test-rebase-inmemory.t
--- a/tests/test-rebase-inmemory.t
+++ b/tests/test-rebase-inmemory.t
@@ -140,12 +140,11 @@
   $ ls -l f | cut -c -10
   -rwxr-xr-x
 
-Rebase the working copy parent, which should default to an on-disk merge even if
-we requested in-memory.
+Rebase the working copy parent
   $ hg up -C 3
   0 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ hg rebase -r 3 -d 0 --debug | grep rebasing
-  rebasing on disk
+  rebasing in-memory
   rebasing 3:753feb6fd12a "c" (tip)
   $ hg tglog
   @  3: 844a7de3e617 'c'