Fasting and stimulated insulin concentrations in four patients with acanthosis nigricans and polycystic ovary syndrome were compared with four patients, matched for weight and ovarian morphology, without acanthosis. The median fasting serum insulin concentrations were 114.2 and 25.1 mU/l in the respective groups (P = 0.02). One additional patient was investigated before and after an 18% increase in weight which resulted in a 170% increase in fasting insulin concentrations and the development of acanthosis nigricans. These observations suggest that there is variation between individuals in the degree of obesity that results in the development of acanthosis and that obese patients with this skin condition represent the severe end of the spectrum of the polycystic ovary syndrome.