Patients with polycystic ovary syndrome (PCO) who are obese and exhibit acanthosis nigricans have been reported to have insulin resistance. We studied 21 euglycemic, euprolactinemic patients with PCO without acanthosis nigricans. Eleven of these patients were hirsute and the remaining 10 were not. Eight women with chronic anovulation, who were weight matched with the PCO patients, were studied along with the 11 ovulatory control subjects. Compared to control subjects, women with chronic anovulation had similar levels of immunoreactive luteinizing hormone (LH), bioactive LH, 17 beta-estradiol, testosterone (T), and dehydroepiandrosterone sulfate but significantly higher fasting immunoreactive insulin levels (p less than 0.05). The nonhirsute patients with PCO had higher immunoreactive insulin levels compared to patients with chronic anovulation and control subjects (p less than 0.05) while hirsute patients with PCO had higher immunoreactive insulin levels than nonhirsute ones (p less than 0.05). There was a significant positive correlation between fasting immunoreactive insulin levels and serum T. Fasting immunoreactive insulin also showed a significant correlation with serum LH and bioactive LH. After receiving 100 gm of oral glucose, hirsute patients with PCO had significantly higher insulin responses compared to those of control subjects despite similar glucose responses. Red blood cell insulin-binding sites and receptor affinities were similar in hirsute patients and control subjects. These data suggest that the hyperinsulinemia of patients with PCO may be related, in part, to the elevated LH and androgen levels and may be a postreceptor defect.