This study was designed to investigate the relationship between insulin resistance and obesity in the pathogenesis of polycystic ovarian syndrome(PCO). Twenty-two women with PCO, of whom thirteen were non-obese with body mass index(BMI, kg/$m^2$) of <25 and nine were obese with BMI${geq}$25 were studied. Eight non-obese control women and seven obese control women were studied. Serum concentrations of testosterone, lutenizing hormone(LH)/follicle-stimulating hormone(FSH) ratio, and insulin-like growth factor I (IGF-I) were found to be significantly higher(P<0.05) in PCO women compared with control women, which clearly is not related to obesity. Serum glucose, insulin, and C-peptide levels were measured during a 2-hour oral glucose tolerance test(OGTT). Non-obese and obese women with PCO both(P<0.05) compared with control women demonstrated significant hyperinsulinemia after OGTT. The degree of hyperinsulinemia was found to be significantly higher in the obese women with PCO compared with the non-obese women with PCO. We concluded that obesity may contribute to hyperinsulinemia, however may not playa central role in the pathogenesis of PCO.