How often do you scoop the box, and how often do you completely clean it? Ideally, it should be scooped twice a day (more often if it needs it) and clean it (dump all the old litter, wash the box--without soap or detergent--and dry it, preferably in the sun, if weather allows), again, more often if needed. That's the first thing to investigate. Next, if that's not a factor, you need to call the vet. A normal, healthy cat should have no odor at all. And such an odor can indicate a health problem. Finally, for a terrific site to help with any kitty problems, go to cats.about.com --I've gotten so much good advice there, for many, many years. And the forum is great, too--though you must register to use it. And it's totally free. Be sure to sign up for Franny Syufy's weekly cat quizzes!