Oreo (bottom) and Ashley (9/13/85-4/11/97) posing for the camera
Ashley died as a result of Chronic Renal Failure, an all-too-common disease of cats. For information about the disease, take a look at the Feline CRF Information Center, an absolutely outstanding site that provides reference materials that have impressed a number of veterinarians who have viewed the site.
There is an online support group for discussion and information exchange about Chronic Renal Failure. The community is called Feline-CRF-Support.