Diabetes mellitus patients have metabolic and endocrine alterations that could contribute to an increased incidence of hormone-related cancers. We assessed the incidence of endometrial and breast cancer among 80,005 women and the incidence of breast cancer among 73,847 men (total of 153,852 patients) identified in the Swedish In-patient Register as having been hospitalized for diabetes mellitus in the period 1965-1983. These patients underwent follow-up through 1989 via the Swedish Cancer Register and other nation-wide Swedish registers. The outcome measures were standardized incidence ratios (SIR) based on age-, sex- and calendar period-specific incidence rates from the entire Swedish population. To minimise the effect of selection bias, we excluded from the calculation of incidence ratios the first year of observation and cases diagnosed incidentally at autopsy. Only first cancers were considered for the estimates. A total of 328 endometrial cancers (SIR = 1.8; 95% confidence interval [CI] = 1.6-2.0), 1,145 female breast cancers (SIR = 1.3; 95% CI = 1.2-1.4) and 13 male breast cancers (SIR = 2.0; 95% CI = 1.0-3.4) were observed. We conclude that compared with the general population, patients with diabetes mellitus have an increased incidence of endometrial and breast cancers.