We took her to the vet a few weeks ago because she stopped eating. The week before that, she peed all over our couch (again) to the point where it soaked through the cushion, the base, and formed a puddle under the couch. We ended up replacing the couch. Luckily, FIL had a couch he was trying to get rid of.
The vet did all the tests and took x-rays, and discovered that one kidney is half the size of the other. The smaller kidney doesn't seem to work anymore. I guess this explains why she drank so much, and peed so much. (hence my interest in karebear's post below about litter.) Now, she is on special renal food to make sure her one functioning kidney stays healthy. Here she is, hanging out and trying to hide from the tyke.

Re: my cat has kidney disease
awww poor thing!
is he on any steriods or anything?
Poor thing! How old is she?
The vet didn't prescribe any steroids, thank goodness. She has a follow up next week, so we'll see how she is doing.
She is about 6. We're not sure her exact age, because she was a stray. She wandered into our lives about 3 years ago, and they guessed she was 3 at the time.
oh no that's young! my cats are between five and seven. i don't want them to start getting sick!