This study analyses data from two nationwide prospective diabetes registries now covering about 3400 cases from 19 million person-years of follow-up in the age group 0-34 years. The risk of developing insulin-dependent diabetes mellitus (IDDM) per 100000 individuals before 15 years was 386 (95% confidence intervals (Cl) : 362–410) for boys and 391 (95% Cl: 367–415) for girls and by 35 years 701 (95% Cl: 671–731) for men and 562(95% Cl: 534–690) for women. The incidence rate showed a maximum for both boys and girls in early puberty. After pubertal years a sharp increase in the male to female incidence ratio of IDDM was notable. At 10–14 years it was 0.94, at 15–19 years 1.59 and at 20–24 years 2.08. A Cox regression model was used to analyse the effects on age at onset of sex, population density and climatological factors as measured by north-south area of residence and season at onset. The effect of sex was confirmed (PP = 0.004) of season was shown when the four seasons were classified according to a four stage scale related to mean temperature. When dividing Sweden into 11 regions according to north-south gradient (Latitude 55°, 56°, 57°, …, 65°) a significant effect (P = 0.038) was also found. However, no effects of population density or living near the coast versus in the interior were found. It is concluded that a large proportion of the young are at risk of developng this chronic disease. In addition to genetic influences the time of disease onset seems to be related to puberty, gender, and climatological and other environmental factors as indicated by a pattern of geographical and seasonal variation.