Measurement of beta-cell function is an important marker of progression to diabetes in individuals at risk for the disease. Although the peak incidence for the disease occurs before 17 years of age, normal values for insulin secretion were not available in this age group. We performed a simplified intravenous glucose tolerance test in 167 normal children, and in 98 islet cell antibody (ICA)-negative and 12 ICA-positive siblings of diabetic patients. Their age range was 1-16 yr. The first phase of insulin secretion, evaluated as the sum of plasma insulin concentrations at 1 and 3 min, increased with age and was significantly lower in ICA-negative siblings (86 +/- 6 microU/ml, P < 0.002) than in normal controls (115 +/- 6 microU/ml). This difference was not apparent before 8 yr of age. None of the ICA-negative siblings developed diabetes after an average of 4.5 yr. ICA-positive siblings at first study had a first phase insulin response similar to that of ICA negative siblings, but significantly lower than that of the normal controls (74 +/- 13 microU/ml, P < 0.02). The reason for the decreased insulin secretion in ICA-negative siblings is unknown, but could involve a defect in the growth of beta-cell mass or insulin secretion that could be part of the multifactorial pathogenesis of type 1 diabetes.