The initial course of Type 1 (insulin-dependent) diabetes mellitus was studied in two groups of Japanese children, i.e. 21 patients with abrupt onset (Group A) and 19 patients detected by urine glucose screening at school with minimal or no symptoms (Group B). There was no statistical difference in mean age at diagnosis between Group A and B (11 +/- 3 years vs 11 +/- 3 years). Group A patients revealed a rapid deterioration of pancreatic B-cell function, but there was evident recovery of the B-cell function from 3 to 9 months following initial treatment. The B-cell capacity in Group B was self maintained until 24 months after diagnosis. Thereafter, even these patients exhibited a progressive decline in the B-cell function. The two groups had a similar incidence of islet cell antibodies at diagnosis (58% vs 69%). However, human leukocyte antigen studies revealed that patients in Group A had a significantly higher prevalence of DR4 and DRW9 than those in Group B (p less than 0.01). These results suggest that in Japanese children there are two forms of diabetes, an abrupt and a slow onset form, which are clinically different and which also seemed to be genetically independent types, or possibly the same disease diagnosed at different stages.