Obesity associates with some degree of insulin resistance (IR). Acanthosis nigricans (AN) is a clinical marker of IR; however, obeses may or may not have AN. Objective: to study in obese patients with AN the body mass index (BMI), oral glucose tolerance curve (OGT), insulinemias and hyperandrogenism. We studied 18 patients aged 10 to 19 (mean 14.2); 10 of whom were females. An OGT was done with 1.75g/Kg body weight; glycemias (GLY) and insulinemias (INS) were measured at 0, 30, 60, 90, 120 and 180 minutes. GLY was measured with the glucose-oxidase peroxidase method; INS was measured with the RIA double antibody method. Presence of hypertension, menstrual cycle alterations, acne and hypertrichosis were registered. AN was classified as mild, moderate or severe by clinical criteria. Peak INS were classified as normal (under 100 uU/ml), mildly (100-200 uU/ml), moderatey (201-300 uU/ml), or severely (over 300 uU/ml) increased. There were 8, 4 and 6 patiens in the mild, moderate and severe categories, respectively. None had the Syndrome of Severe Insulin Resistance type A. All had a normal OGT, except for one case with diabetes curve. BMI was 25-29 (9 patients), 30-40 in 6 and over 40 in 3. Only 3 of the 10 girls were hyperandrogenic, though only those with hypertrichosis, virilization, acne or menstrual abnormalities were tested. Though we did established IR in these obese patients with AN, we did not find a direct correlation between the degree of BMI, AN and IR. The follow-up of these young people will permit us to evaluate a possible progression to diabetes mellitus type II if their IR is not corrected, or to the polycystic ovary syndrome. We suggest to search for acanthosis nigricans in all obese patients.