Incidence data from Denmark on the insulin-dependent diabetes mellitus in the age group 0--29 years have been collected in two different geographical areas (West and South Jutland plus Copenhagen and North Zealand). An incidence of 13.3 per 100,000 per year (range 12.5--13.6) was registered (N = 792). No difference was found between the two areas. The male incidence exceeded the female incidence by 25.4 per cent. The age distribution showed rising values until a peak in the early puberty with a decline until a rather constant level after puberty. From year to year a seasonal variation of onset was demonstrable with maximum in the winter and minimum in the summer for males only. Ascertainment was found to be between 87.7 and 98.6 per cent and the annual number of new insulin-dependent diabetics in Denmark in the age group 0--29 years can be calculated to 310-350.