The existence of islet cell antibodies (ICA) strongly supports an autoimmune pathogenetic mechanism for insulin-dependent diabetes. ICA have been determined in a group of 205 diabetics with the following results: ICA were present in 76 percent of insulin-dependent diabetics with symptomatology lasting less than 8 weeks, in 20 percent of the remaining insulin-dependent diabetics, in 5 percent of non-insulin-dependent diabetics, and in 1 percent of the control subjects. Thyroid antimicrosome antibodies and antiparietal-cell antibodies were determined too, but no significant relationship was demonstrated in insulin-dependent diabetics with positive ICA. Under a clinical point of view the comparison between diabetics with and without ICA has been of no value for determining differential characteristics in relation to metabolic seriousness, epidemiological data, and complication rate.