I Replaced The Cartridge Of A Price Pfister Bathtub Faucet Because It Was Leaking. Now The Hot And Cold Water Are Opposite. There Is Only One Way To Install To Make The Handle Go Off And On. What Is Wrong?

Nice answer, call a plumber. I just replace my cartridge and had the same issue. Very simple solution is to rotate the valve stem 180 degrees. This is the part held on with 4 screws. The cartridge itself is universal any direction. It's the valve stem that controls the hot and cold sides.

If the faucet was purchased after 1997 call 1 800 PFAUCET for free replacement of cartridge. It's not too difficult to replace. On the Hot/Cold question, if you look inside the back end of the cartridge you may notice two size openings, I suspect if you take it apart and turn the cartridge around it will fix the problem. It's definitely NOT related to the (single?) handle.

You should be able to remove the cartridge and re-install it 180degrees rotated from where it was.