Islet cell antibodies (ICA), complement fixing islet cell antibodies, immune complexes and thyro-gastric autoantibodies were studied in newly diagnosed diabetic patients not requiring insulin at diagnosis. Particular attention was focussed on that minority of patients who are initially treated with diet or oral agents but show ICA in their serum. One hundred and six non-insulin-requiring patients were studied at clinical diagnosis. Seventeen who had ICA in their serum were compared with a control group of 89 who did not. The 17 ICA-positive diabetic patients were followed serologically for approximately 1 year from diagnosis. Patients were followed clinically for 3 years. Forty-seven percent of ICA-positive and 19% of ICA-negative patients had immune complexes in their serum. Eleven of the 17 ICA-positive patients also had serum complement fixing islet cell antibodies. Thyro-gastric antibodies were found in 29% of ICA-positive and 18% of ICA-negative diabetic patients. ICA, complement fixing antibody and immune complex positivity declined with time. Ten of the 7 ICA-positive and two of the 89 ICA-negative patients required insulin within 3 years of diagnosis. There was a positive trend for the presence of complement fixing islet cell antibodies at diagnosis to be associated with the early development of insulin dependency. The type of diabetes in ICA-positive patients not requiring insulin at diagnosis has strong immunological and clinical similarities to classical Type 1 (insulin-dependent) diabetes.