Fifty-three children of insulin dependent (IDM) and 20 children of gestational diabetic mothers (IGDM) representing 80 and 91%, respectively, of all surviving infants of diabetic mothers born between 1969 and 1972 at Sabbatsberg's hospital, Stockholm, participated in the follow-up study. The first follow-up examination was performed when the children had reached approximately 5 years of age and included measurement of height and weight, insulin response to intravenous glucose, and HLA typing. When the children were approximately 11 years old, a search was performed in the national register for type I diabetes in children in order to ascertain the frequency of type I diabetes mellitus in the total series (n = 88). The majority of children had a normal height for age and desirable weight for height. At the first follow-up all children had normal glucose disappearance rates (kt) and the insulin response including the early insulin response to glucose were not different between IDM and IGDM groups. The frequency distribution of HLA antigens (A, B, C) was not different from normal and there was no association between HLA B 8 and/or B 15 and early insulin response or kt values. At the second follow-up, two children of type I diabetic mothers had acquired type I diabetes, both were HLA B 15 positive, had normal kt values at the first follow-up, one with low, one with a high early insulin response. The frequency of type I diabetes in offspring of insulin dependent diabetic mothers was 3%.