The objectives of the study were to identify recent trends in the prevalence of diabetes mellitus (DM) in dogs and to identify host risk factors. Veterinary Medical Data Base (VMDB) electronic records of 6860 dogs with a diagnosis of DM (VMDB code 870178500) between 1970 and 1999 were evaluated to determine time trends. Records of 6707 dogs with DM and 6707 frequency matched dogs with any diagnosis other than DM from the same teaching hospitals in the same year, selected as controls, were evaluated for risk factor analysis. The prevalence of DM in dogs presented to veterinary teaching hospitals increased from 19 cases per 10,000 admissions per year in 1970 to 64 cases per 10,000 in 1999, while the case-fatality rate decreased from 37% to 5%. The hospital prevalence of DM was consistently greater over time in older compared with younger dogs with the highest prevalence occurring in dogs 10-15 years of age. Dogs weighing <22.7 kg had a significantly (P<0.001) greater risk of DM compared with heavier dogs. Female dogs had an increased risk of DM compared with males (P<0.001).