"rhw" wrote in message
...
I have two 3-way diimmer switches, one at the top of stairs and one at
the bottom that control lights at both the top and bottom of the
stairs. If the switch at the bottom of the stairs is in the "on"
position (up) the switch at the top of the stairs will turn the lights
on and off. If the switch at the bottom of the stairs is in the "off"
position (down) the switch at the top of the stairs will not work.
I'm assuming one of the switches is bad, but which one do I replace?

If it used to work, then one (or both) of the switches has probably failed.
But, if it has never worked then the more likely cause of what you
describe is the wires reversed on the upstairs switch.