This is a prospective study of the incidence of insulin-dependent diabetes mellitus (IDDM) in children 0-14 years of age, including all newly diagnosed cases in the whole of Sweden from July 1, 1977 until June 30, 1980. All 45 Swedish departments of paediatrics participated. During the three-year-period studied, 1108 Swedish children, 0-14 years of age had their onset of diabetes. That means around 369 new diabetics yearly in the age groups studied. The mean yearly incidences in the years 1977-80 were 22.6, 22.8 and 22.6 per 100000 children, respectively. Mean prevalence on June 30, 1980 and 1.48 per 1000 children 0-14 years with a wide range of 0.71-2.65. The age distribution at onset showed a gradual increase and peak incidences at 11 years of age for the girls and 4 and 13 years of age for the boys. There was a consistently higher incidence for boys in the younger age groups during the three-year-period studied. Peak incidences of new cases were reached in January, March and July through October for the age groups 5-9 and 10-14 years of age. No such seasonal variation was seen for children 0-4 years of age. The cumulative incidence of IDDM at 14 years of age was 3.2 per 1000 for the boys and 2.9 per 1000 for the girls. The degree of ascertainment in this study was 93.4%.