The patient, a female 6 years 4 months old, diagnosed as affected by Acanthosis nigricans and diabetes mellitus was referred to our Clinic for further investigation of her glucose metabolism. She had a typical face with hypertelorism, prognathism, macroglossia and large auricles. The skin was hyperpigmented, verrucous, hyperkeratotic especially in the folds and flexural areas with small papillomatous or nodular growth; diffused hypertrichosis and hypertrophic clitoris were also present. Biochemical and hormonal investigations revealed no major abnormalities apart from glucose metabolism. After an unsuccessful trial with conventional insulin therapy, endovenous continuous insulin infusion was started: even with 32 U/kg/h it was not possible to achieve normoglycemia. Insulin receptors were studied on erythrocytes: 125I-insulin binding (specific) was clearly lower than normal. The concentration of insulin receptors was reduced, while the average affinity profile was normal. The study of erythrocyte insulin receptors has demonstrated that insulin resistance in this patient is due to a decrease in the number of receptors, i.e. Acanthosis nigricans type A.