We present data on 5 aspects of the epidemiology of diabetes mellitus in Canada: (a) the incidence of insulin-dependent diabetes mellitus in those under 15 years of age. The 2 Canadian centres that participated in the Diabetes Epidemiology Research International study had different incidence rates in IDDM:25.5/100,000 in Prince Edward Island (PEI) and 9.2/100,000 in Montreal. The reasons for this difference are not yet established. Studies on incidence of IDDM over a decade in PEI showed an apparent epidemic of the disease; (b) the prevalence of self-reported diabetes mellitus in Canadian adults. The overall prevalence of self-reported diabetes in Canadian adults (18-74 y) was 5.1% in the Canadian Heart Health Survey. There were no significant regional differences in prevalence of diabetes across Canada. The prevalence rates increased with age; (c) mortality data in people with diabetes mellitus. In PEI, 321 persons with diabetes died between January 1, 1982 and December 31, 1984, accounting for about 2% of all deaths. Diabetes was listed as the underlying cause in 16.8% of the deaths, as a contributing cause of death in 41.7%, and not mentioned at all in 41.1% of the deaths. Irrespective of whether diabetes was mentioned or not, myocardial infarction and cerebral vascular disease were the 2 major causes of deaths in these 321 persons with diabetes; (d) the prevalence of cardiovascular risk factors in Canadian adults with diabetes mellitus. In the Canadian Heart Health Survey, the prevalence rates of obesity, hypertension, sedentary lifestyle, and hypercholesterolemia were higher in the diabetic group.(ABSTRACT TRUNCATED AT 250 WORDS)