A 64-year-old black man presented with the syndrome of acanthosis nigricans and insulin-resistant diabetes mellitus requiring up to 3000 units of insulin per day. The patient's plasma contained circulating antibodies to insulin receptors thought to be responsible for the insulin resistance. The marked insulin resistance, the manifestations of acanthosis nigricans, the evidence of immunologic dysfunction by the absence of expected circulating antibodies to insulin, and the demonstration of circulating antibodies to insulin receptors put this patient in Kahn's category B of insulin resistance and acanthosis nigricans. There was no evidence of malignancy, lipodystrophy, or endocrine abnormality. The occurrence of acanthosis nigricans with insulin resistance due to binding of cell membrane insulin receptors by antibodies has been reported exclusively in women. This case report is the first description of a male patient with the syndrome of insulin resistance and acanthosis nigricans and focuses attention on features that might mislead one to suspect other causes of insulin resistance.