Acanthosis nigricans (AN) with insulin resistance has been traditionally attributed to insulin receptor abnormalities. To further clarify the postbinding defects of in vivo insulin action in this state, we applied the euglycemic insulin clamp technique, combined with the glucose trace infusion method, to 26 subjects: 12 AN patients (eight normoglycemic and four hyperglycemic), eight obese, and eight lean control subjects. The normoglycemic AN group exhibited fasting hyperinsulinemia (666% of control), 160% elevated hepatic glucose production (HGP), 425% increased posthepatic insulin delivery rate, and only slightly reduced (19%) insulin clearance rates, compared with controls. Except for the latter, all these abnormalities were statistically significant (P less than .05), and could not be accounted for by body overweight. AN patients with diabetes mellitus (AN + DM) exhibited a further decreased insulin responsiveness (30%) and clearance (38%), together with a major increase in HGP (320%). All AN patients showed a significant right-shift in the insulin dose-response curve, indicating a decrease in insulin sensitivity. In conclusion, AN is characterized by increased basal rates of HGP, and peripheral insulin resistance, which can be partially attributed to postbinding defects. In AN + DM, a worsening of these abnormalities may be responsible for unmasking the existence of diabetes.