The incidence of type I (insulin-dependent) diabetes was determined in the district of Catania (eastern Sicily) in children under 15 years of age over a 3-year period (1 January 1989 to 31 December 1991). Two independent sources of information were used. The primary source was contact with all medical services in the province, and the secondary source was the personal identification cards issued to all diabetic patients by the National Health System necessary for obtaining free medical care. The information obtained was 99.6% complete. The overall incidence was 10.2/10(5) per year. This study provides the first standardized data on the incidence of type I diabetes in Sicily and is consistent with the possibility of regional deviations from the proposed north to south gradient.