The hospital records of 477 patients under 18 years of age with insulin-dependent diabetes mellitus (IDDM) who were followed in the Endocrinology Unit of Hacettepe University Children's Hospital between the years 1969 and 1991 were analyzed for age, sex, residence at time of diagnosis, date of onset of symptoms, date of diagnosis, family history of IDDM, and consanguinity between parents. The distribution of age at diagnosis showed a small peak between 4 and 6 years of age and a main peak between 12 and 14 years. In girls, the main peak appeared between 10 and 12 years, and in boys between 12 and 14. A significant difference was not seen between sexes (239 males and 236 females). The frequency of diagnosis showed seasonal variations the lowest in autumn and the highest in winter. Consanguinity between parents was 23.9 percent, and 10.3 percent of the patients had IDDM in first degree relatives.