To clarify the heterogeneity of Type 1 (insulin-dependent) diabetes mellitus, differences between patients with different HLA risk antigens were investigated with regard to sex, age at diagnosis, season of year and calendar year at diagnosis of the disease. The study consisted of 293 HLA-typed patients from the Department of Paediatrics, University of Oulu, Oulu, Finland. HLA-Dw2 was extremely rare among diabetic patients, whereas Dw3 and Dw4 were associated with increased risk in this as in other series. Male patients more often had the HLA-A1 antigen than females. On comparison of the Dw3 positive patients, boys more frequently had the combination A1,B8 than girls. A1,B8-positive patients were more often diagnosed during the warm months, in the late summer and autumn. Patients with both Dw3 and Dw4 were younger at diagnosis when compared with the rest of the patients. The results support the concept of heterogeneity in the pathogenesis of Type 1 diabetes associated with HLA-linked genetic determinants.