For an epidemiological study of childhood diabetes, application forms for medical benefits filed in Osaka Prefecture were collected and analyzed. A total of 465 patients with childhood diabetes under 18 yr of age were identified during the period between 1978 and 1988; 371 (175 boys and 196 girls) of them were classified as insulin-dependent diabetes mellitus (IDDM), and the others were either non-insulin-dependent diabetes mellitus (NIDDM) or an unidentified type of diabetes. The mean annual incidence of IDDM was 1.68 for boys and 2.00 for girls per 100,000 population, the rate being slightly higher for girls. The rates increased among older children, with a peak in the 10-14-yr age group. The prevalence estimated on the basis of the cumulative incidence in 1988 was 1.29 for boys and 1.60 for girls per 10,000 population, indicating a marked increasing trend with age. A significant seasonal variation was observed in the months of onset for IDDM, with higher rates during the winter. The mean birth weights of the male and female patients with IDDM were slightly lower than the national means. Use of official documents obtained from the medical benefits system was found to be an effective method for identification of cases of IDDM in the community, although further information is required for more accurate confirmation of such cases.