Sera of 4208 schoolchildren in Ulm and surrounding areas (1916 boys, 2292 girls; mean age 13.9 [7-21] years) were analysed for the presence of cytoplasmatic islet-cell antibodies (ICA). ICA were demonstrated in 44 children (1.05%). Complement-fixing ICA (CF-ICA) were found in six of them, insulin autoantibodies (IAA) in seven. The early phase of insulin response in the intravenous glucose tolerance test was below the 1st percentile of normal controls in two of the ICA-positive children. In the course of the subsequent 6-9 months the ICA disappeared in half of the previously positive children. In the two with abnormal glucose tolerance tests ICA, CF-ICA and abnormal insulin secretion persisted during the observation period of 9 months. In another CF-ICA-positive schoolchild insulin response to the glucose tolerance test fell below the 1st percentile after 6 months. These results indicate that, even in an unselected group of children, persisting ICA and CF-ICA are high-risk markers for abnormal insulin secretion, which precedes the onset of type I diabetes.