Islet cell antibodies were found in 71 of 1169 first-degree relatives (6.1%) from 448 families who had a proband with type I diabetes. Seven children have since become insulin dependent. All had islet cell antibodies and were followed up prospectively with measurement of first-phase insulin production during intravenous glucose tolerance testing. In this group the statistical probability of developing type I diabetes within 12 months with 95% confidence was found to be 59% to 100% when the first-phase insulin secretion was less than 25 microU/mL. The identification of the prediabetes time period should allow an opportunity for intervention in the underlying disease process to determine if the onset of type I diabetes can be altered.