We investigated a family in which at least 4 men in 3 generations had a syndrome of obesity, mild mental retardation, delayed puberty, macroorchidism, acanthosis nigricans, hyperinsulinemia, and later overt insulin-resistant diabetes mellitus (non-insulin-dependent diabetes mellitus, NIDDM). The patients have markedly curly scalp hair, deficient face and body hair. Their teeth were healthy and normal in size and position. The clinical and biochemical findings and characteristics of the insulin receptors investigated in fibroblasts are reported. There was normal insulin binding to fibroblasts in the 2 brothers and their father. However, insulin-stimulated RNA synthesis was decreased as compared to that of normal control individuals. These findings suggest a postbinding defect of insulin action. The pedigree documents an autosomal dominant mode of inheritance. The diagnosis is of practical importance since it enables medical supervision of gene carriers in a preclinical state of atherosclerotic complications and overt diabetes. The findings in this family have relevance also to the explanation of familial mild mental retardation and to the study of different forms of insulin resistance due to a disturbance in biosignal transfer.