diff --git a/doc/hgrc.5.txt b/doc/hgrc.5.txt
--- a/doc/hgrc.5.txt
+++ b/doc/hgrc.5.txt
@@ -377,6 +377,9 @@ ui::
     remote command to use for clone/push/pull operations. Default is 'hg'.
   ssh;;
     command to use for SSH connections. Default is 'ssh'.
+  strict;;
+    Require exact command names, instead of allowing unambiguous
+    abbreviations.  True or False.  Default is False.
   timeout;;
     The timeout used when a lock is held (in seconds), a negative value
     means no timeout. Default is 600.
diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -507,7 +507,7 @@ def help_(ui, name=None, with_version=Fa
         if with_version:
             show_version(ui)
             ui.write('\n')
-        aliases, i = findcmd(name)
+        aliases, i = findcmd(ui, name)
         # synopsis
         ui.write("%s\n\n" % i[2])
 
@@ -1154,7 +1154,7 @@ def debugcomplete(ui, cmd='', **opts):
         options = []
         otables = [globalopts]
         if cmd:
-            aliases, entry = findcmd(cmd)
+            aliases, entry = findcmd(ui, cmd)
             otables.append(entry[1])
         for t in otables:
             for o in t:
@@ -1164,7 +1164,7 @@ def debugcomplete(ui, cmd='', **opts):
         ui.write("%s\n" % "\n".join(options))
         return
 
-    clist = findpossible(cmd).keys()
+    clist = findpossible(ui, cmd).keys()
     clist.sort()
     ui.write("%s\n" % "\n".join(clist))
 
@@ -3149,7 +3149,7 @@ norepo = ("clone init version help debug
           " debugindex debugindexdot")
 optionalrepo = ("paths serve debugconfig")
 
-def findpossible(cmd):
+def findpossible(ui, cmd):
     """
     Return cmd -> (aliases, command table entry)
     for each matching command.
@@ -3162,7 +3162,7 @@ def findpossible(cmd):
         found = None
         if cmd in aliases:
             found = cmd
-        else:
+        elif not ui.config("ui", "strict"):
             for a in aliases:
                 if a.startswith(cmd):
                     found = a
@@ -3178,9 +3178,9 @@ def findpossible(cmd):
 
     return choice
 
-def findcmd(cmd):
+def findcmd(ui, cmd):
     """Return (aliases, command table entry) for command string."""
-    choice = findpossible(cmd)
+    choice = findpossible(ui, cmd)
 
     if choice.has_key(cmd):
         return choice[cmd]
@@ -3215,7 +3215,7 @@ def parse(ui, args):
 
     if args:
         cmd, args = args[0], args[1:]
-        aliases, i = findcmd(cmd)
+        aliases, i = findcmd(ui, cmd)
         cmd = aliases[0]
         defaults = ui.config("defaults", cmd)
         if defaults:
diff --git a/tests/test-strict b/tests/test-strict
new file mode 100755
--- /dev/null
+++ b/tests/test-strict
@@ -0,0 +1,19 @@
+#!/bin/sh
+
+hg init
+
+echo a > a
+hg ci -d '0 0' -Ama
+
+hg an a
+
+HGRCPATH=$HGTMP/.hgrc; export HGRCPATH
+echo "[ui]" >> $HGTMP/.hgrc
+echo "strict=True" >> $HGTMP/.hgrc
+
+hg an a
+hg annotate a
+
+echo % should succeed - up is an alias, not an abbreviation
+
+hg up
diff --git a/tests/test-strict.out b/tests/test-strict.out
new file mode 100644
--- /dev/null
+++ b/tests/test-strict.out
@@ -0,0 +1,26 @@
+adding a
+0: a
+hg: unknown command 'an'
+Mercurial Distributed SCM
+
+basic commands (use "hg help" for the full list or option "-v" for details):
+
+ add        add the specified files on the next commit
+ annotate   show changeset information per file line
+ clone      make a copy of an existing repository
+ commit     commit the specified files or all outstanding changes
+ diff       diff repository (or selected files)
+ export     dump the header and diffs for one or more changesets
+ init       create a new repository in the given directory
+ log        show revision history of entire repository or files
+ parents    show the parents of the working dir or revision
+ pull       pull changes from the specified source
+ push       push changes to the specified destination
+ remove     remove the specified files on the next commit
+ revert     revert files or dirs to their states as of some revision
+ serve      export the repository via HTTP
+ status     show changed files in the working directory
+ update     update or merge working directory
+0: a
+% should succeed - up is an alias, not an abbreviation
+0 files updated, 0 files merged, 0 files removed, 0 files unresolved