diff --git a/mercurial/win32.py b/mercurial/win32.py
--- a/mercurial/win32.py
+++ b/mercurial/win32.py
@@ -119,6 +119,27 @@ class _CONSOLE_SCREEN_BUFFER_INFO(ctypes
 
 _STD_ERROR_HANDLE = _DWORD(-12).value
 
+# CreateToolhelp32Snapshot, Process32First, Process32Next
+_TH32CS_SNAPPROCESS = 0x00000002
+_MAX_PATH = 260
+
+class _tagPROCESSENTRY32(ctypes.Structure):
+    _fields_ = [('dwsize', _DWORD),
+                ('cntUsage', _DWORD),
+                ('th32ProcessID', _DWORD),
+                ('th32DefaultHeapID', ctypes.c_void_p),
+                ('th32ModuleID', _DWORD),
+                ('cntThreads', _DWORD),
+                ('th32ParentProcessID', _DWORD),
+                ('pcPriClassBase', _LONG),
+                ('dwFlags', _DWORD),
+                ('szExeFile', ctypes.c_char * _MAX_PATH)]
+
+    def __init__(self):
+        super(_tagPROCESSENTRY32, self).__init__()
+        self.dwsize = ctypes.sizeof(self)
+
+
 # types of parameters of C functions used (required by pypy)
 
 _kernel32.CreateFileA.argtypes = [_LPCSTR, _DWORD, _DWORD, ctypes.c_void_p,
@@ -186,6 +207,15 @@ except AttributeError:
 _user32.EnumWindows.argtypes = [_WNDENUMPROC, _LPARAM]
 _user32.EnumWindows.restype = _BOOL
 
+_kernel32.CreateToolhelp32Snapshot.argtypes = [_DWORD, _DWORD]
+_kernel32.CreateToolhelp32Snapshot.restype = _BOOL
+
+_kernel32.Process32First.argtypes = [_HANDLE, ctypes.c_void_p]
+_kernel32.Process32First.restype = _BOOL
+
+_kernel32.Process32Next.argtypes = [_HANDLE, ctypes.c_void_p]
+_kernel32.Process32Next.restype = _BOOL
+
 def _raiseoserror(name):
     err = ctypes.WinError()
     raise OSError(err.errno, '%s: %s' % (name, err.strerror))
@@ -309,6 +339,51 @@ def termwidth():
     width = csbi.srWindow.Right - csbi.srWindow.Left
     return width
 
+def _1stchild(pid):
+    '''return the 1st found child of the given pid
+
+    None is returned when no child is found'''
+    pe = _tagPROCESSENTRY32()
+
+    # create handle to list all processes
+    ph = _kernel32.CreateToolhelp32Snapshot(_TH32CS_SNAPPROCESS, 0)
+    if ph == _INVALID_HANDLE_VALUE:
+        raise ctypes.WinError
+    try:
+        r = _kernel32.Process32First(ph, ctypes.byref(pe))
+        # loop over all processes
+        while r:
+            if pe.th32ParentProcessID == pid:
+                # return first child found
+                return pe.th32ProcessID
+            r = _kernel32.Process32Next(ph, ctypes.byref(pe))
+    finally:
+        _kernel32.CloseHandle(ph)
+    if _kernel32.GetLastError() != _ERROR_NO_MORE_FILES:
+        raise ctypes.WinError
+    return None # no child found
+
+class _tochildpid(int): # pid is _DWORD, which always matches in an int
+    '''helper for spawndetached, returns the child pid on conversion to string
+
+    Does not resolve the child pid immediately because the child may not yet be
+    started.
+    '''
+    def childpid(self):
+        '''returns the child pid of the first found child of the process
+        with this pid'''
+        return _1stchild(self)
+    def __str__(self):
+        # run when the pid is written to the file
+        ppid = self.childpid()
+        if ppid is None:
+            # race, child has exited since check
+            # fall back to this pid. Its process will also have disappeared,
+            # raising the same error type later as when the child pid would
+            # be returned.
+            return " %d" % self
+        return str(ppid)
+
 def spawndetached(args):
     # No standard library function really spawns a fully detached
     # process under win32 because they allocate pipes or other objects
@@ -339,7 +414,8 @@ def spawndetached(args):
     if not res:
         raise ctypes.WinError
 
-    return pi.dwProcessId
+    # _tochildpid because the process is the child of COMSPEC
+    return _tochildpid(pi.dwProcessId)
 
 def unlink(f):
     '''try to implement POSIX' unlink semantics on Windows'''