We screened 1,000 individuals for pre-type I diabetes mellitus: 927 were first degree relatives of patients treated at our institution, 31 other had related autoimmune diseases, 42 had fortuitously discovered slight hyperglycemia. Islet cell antibodies were present in 31 subjects, 13 adults and 18 children. Among these 18 children, 7 also had complement-fixing islet cell antibodies, and 5 anti-insulin antibodies. All but one initially had a normal insulin response to intravenous glucose. The child with the abolished response, as well as another whose insulin secretion fell down during the study period, became diabetic. Based on these preliminary data, we are developing large scale family screening of pre-type I diabetes.