Medical records from 333 cats with diabetes mellitus were studied retrospectively, using epidemiologic methods to determine the incidence of and risk factors for diabetes mellitus in this species. Abstracts were derived, using the Veterinary Medical Data Program with its 17 participating academic institutions in the United States and Canada. A reference population of 135,651 cats was derived from the same hospital population and time span (July 1980 to June 1986). The incidence of diabetes mellitus in cats was determined to be 2.45 cases/1,000 cat-years-of-risk during the 6-year study period. Breed had no detectable effect on risk for diabetes mellitus. In contrast, body weight, age, gender, and neutering had a significant (P less than or equal to 0.01) effect. Body weight of cats was categorized as being less than or greater than or equal to 6.8 kg. The higher body weight, probably indicating obesity, contributed a 2.2-fold increase in risk, even after adjustment for age and gender (adjusted odds ratio). The etiologic fraction for high body weight was 3.8%, suggesting that an estimated 3.8% of cases of diabetes mellitus was attributable to this factor alone. Over 50% of diabetic cats were greater than 10 years old, and the etiologic fraction for age greater than 7 years alone was 73.5%. Age was a significant (P less than 0.001) and the most important single risk factor for development of the disease in cats, with adjusted odds ratios of 8.3 and 14.4 for age 7 to 10 years and greater than 10 years, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)