Take the cat to the vet. It could be an infection, or it could be a behavioral problem. The former is fixed with medication, the latter is fixed through a few potetial ways.

-is the litter box cleaned regularly?
-is the old litter regularly dumped and new put in the box?
-multiple cats = multiple boxes. one extra box for each additional cat.
-try putting a little box in a new location. you can't put one on a couch, but, this was how I solved a similar problem for my cats.


At any rate, take the cat to the vet if you haven't already.
Good luck.