Insulin-dependent diabetes mellitus (IDDM) was studied retrospectively in children 0-14 years of age registered at a northern Swedish department of Paediatrics. In the years 1938-77, the new cases of IDDM registered were 307. The mean yearly incidences were 14.2 cases per 100,000 during 1938-42 and 37.9 cases per 100,000 during 1973-77. The lowest incidence rate of 10.2 per 100,000 was seen during the wartime, 1940-45. To investigate the changing age distribution, directly standardized rate ratios (SRR) were calculated for 4 ten-year periods. Significantly lower incidence (SRR = 73) was observed during the first period 1938-47 and significantly higher (SRR = 137) for the last period 1968-77 as compared to the whole period (SRR = 100). The increasing incidence during the last decade was especially marked for children below 5 years of age. The geographic variation between 11 community areas was measured by the indirectly standardized morbidity ratios (SMR) showing a range between 40 and 179. Peak incidences of new cases were seen during winter and summertime. The current (31 December 1977) prevalence of IDDM among children 0-14 years of age in Västerbotten was 3.0 per 1,000. Our study suggests that the incidence of insulin-dependent diabetes among children has been increasing since 1938.