A retrospective study was undertaken to estimate the incidence of diabetes mellitus in Norwegian children. Data were collected from all hospitals in the country and from a central insurance register. Eight hundred and forty-five new cases in the age group 0-14 years and with onset in the five-year study period 1973-1977 were detected. The calculated mean annual incidence was 17.6 per 100 000 children, with a year-to-year variation of 15.4-19.3 per 100 000. The geographic variation in incidence was considerable with the lowest rate in the North (6.8/100 000/year) and the highest rates in the South-Eastern part of the country (approx. 20/100 000/year). There was a significantly higher incidence for boys (18.8/100 000/year) than for girls (16.4/100 000/year). The age variation in the incidence rates showed rising values towards a peak at 12 years for girls and a plateau at 12-14 years for boys, with an abrupt decline after 12 and 14 years, respectively. There were more cases with onset in the winter and autumn, with significant peaks in February and October. From these data the prevalence of diabetes mellitus in the age group 0-14 years can be calculated to 1.2 per 1 000 children. In the whole of Norway, about 170 new cases of diabetes mellitus below the age of 15 years can be expected every year. Compared with previous studies, the present data suggest an increasing incidence of childhood diabetes in Norway.