All newly diagnosed cases of diabetes mellitus aged 15-34 years in Sweden, where the population in this age interval is about 2.3 million, were registered on standardized forms. During 1983, the first year of the study, there were 311 males and 161 females, excluding 280 with gestational diabetes. The annual incidence of diabetes was 26.2 per 100,000 in males and 14.2 in females. The respective figures for type I were 18.5 and 10.1, and for type II 5.7 and 2.9. The incidence of type I diabetes was similar for the four age groups (15-19, 20-24, 25-29, 30-34 years), while for type II it was highest in the oldest group. Types I and II, but not the sexes, differed as regards the cumulative distribution curves of the maximum blood glucose concentration during the first two weeks after diagnosis. The present incidence of diabetes in Sweden is higher, particularly in males than the rates for similar age groups in Oslo (1925-64) and Denmark (1970-77).