The frequency of diabetes mellitus is described for cats that received veterinary care from two large feline-only clinics in Brisbane, Australia. Frequency was estimated using period prevalences (the proportion of the population at risk that was affected by diabetes at any point during a specified time period). Of the 12,576 study cats, 93 were affected with diabetes during the 5-year study period, resulting in a 5-year period prevalence of 7.4 per 1000 cats. Period prevalence was significantly higher in Burmese cats (22.4 cats per 1000) than domestic short and longhaired cats (7.6 cats per 1000) and the mean age at first diagnosis during the study period was significantly higher amongst Burmese cats (13.6 years) compared to domestic short and longhaired cats (10.9 years). Further investigations into the apparent predisposition of Burmese cats to diabetes mellitus are indicated.