In 868 insulin-treated diabetic children and adolescents with onset of IDDM under age 20 we investigated the frequency of IDDM and NIDDM in all first-degree relatives. On the basis of the National Diabetes Register of the GDR the age-corrected lifetime risk for the development of IDDM and NIDDM was calculated for the general population and for parents and siblings of diabetic children. The age-corrected risk for IDDM, but not for NIDDM, is statistically significantly higher in fathers (10.26 +/- 1.75%) than in mothers (5.28 +/- 1.49%) and is about equal in brothers (30.71 +/- 6.07%) and sisters (35.54 +/- 6.28%) of children with IDDM. Among general population the age-corrected life-time risk for IDDM is equal for males (4.35 +/- 0.02%) and females (4.91 +/- 0.02%), but is significantly higher for NIDDM in females (27.49 +/- 0.04% contrary to 24.07 +/- 0.05%). In comparison with the data of Tillil and Köbberling (1987) our lifetime risk estimates show a shifting of risk for IDDM and NIDDM into older age groups.