The objective of the study was to determine the incidence and estimated median time to cataract formation in dogs with diabetes mellitus. The animals studied were 200 dogs with diabetes mellitus which were referred to a university teaching hospital between 1985 and 1995. Medical records from dogs with a diagnosis of diabetes mellitus were reviewed and, where necessary, further follow-up information was gathered from the referring veterinarian. Incidence rate and median time to diabetic cataract formation was calculated using survival-analysis techniques in a retrospective cohort study design. Among the 200 dogs in the study population, 23 had cataracts at the time of diabetes diagnosis that were presumed to be related to other disease processes. Of the remaining 177 dogs, 132 had documented cataract development with features suggestive as being secondary to diabetes. Twenty-three dogs did not have obvious cataracts at the time of their last examination while 22 dogs did not have cataracts at the time they were lost to follow-up. These 55 cases contributed to the statistical models as noncases of cataracts until the last date for which an examination was available. Half of the population had developed cataracts by the 170th day postdiagnosis of diabetes mellitus, while 75% and 80% of the population developed cataracts by 370 days and 470 days, respectively. The results of this study suggest that the majority of dogs with diabetes will develop cataracts within 5-6 months from the time of diagnosis of the disease, and that approximately 80% of dogs will develop cataracts within 16 months of diagnosis.