Diabetes mellitus is a genetically determined disorder of metabolism in which inherited susceptibility plays an important part. We studied the distribution of HL-A antigens, and theincidence of lymphocytotoxic and tissue antibodies in seventy-one adult diabetic patients—fifty insulin-dependent and twenty-one insulin-independent. Specificity W15 was present in significantlyhigher frequency in the insulin-dependent diabetic patients than either in the control group (P = 0.0005) or in the insulin-independent diabetic patient group (P < 0.0005). The incidence of lymphocytotoxic and tissue antibodies in the two patient groups was not different from that in the control population.