In order to establish the role of insulin in the pathogenesis of lipid abnormalities in hyperandrogenic women with the polycystic ovary syndrome (PCO) 49 women aged 18 to 35 yr with a normal glucose tolerance test were studied. They were divided into two groups: 27 women with PCO (9 obese and 18 nonobese), and 22 healthy women (12 with simple obesity and 10 with normal body weight). In the PCO group, the fasting insulin levels and the insulin response to oral glucose load were higher than in the matched controls. Significantly lower levels of HDL2-cholesterol and higher levels of apolipoprotein B were observed in obese and non nonobese PCO patients. In obese women with PCO this was associated with lower levels of HDL-cholesterol and apolipoprotein A-I (Apo A-I), whereas the levels of total triglycerides and VLDL-triglycerides (VLDL-TG) were increased. Multiple regression analysis in PCO women, after adjustment for age, body mass index and the levels of insulin and sex hormones, showed a strong positive correlation between the fasting insulin levels and total triglycerides and VLDL-TG, while a negative correlation was found between fasting insulin levels and apo A-I. These results indicate that hyperinsulinemia may play a role in the development of lipid disturbances in women with the PCO.