Polycystic ovarian syndrome (PCO) occurs frequently in hirsute patients. A dissociated explosive response of LH to LHRH administration has been associated with the diagnosis of PCO. Twenty-four of 58 women seen because of hirsutism were found to have ovarian dysfunction based on clinical signs such as anovulation and irregular menstrual cycles. Plasma androgen levels were elevated in the patient group. The LHRH test (200 micrograms, iv) was applied to the 24 patients and compared with 13 normal ovulatory controls. Serum FSH levels before and after the LHRH test were normal in all patients. Two patterns of LH response to LHRH stimulation were observed: an explosive response in 17 patients (delta LH: 39.4 +/- 21.8 IU/l, control group: 7.35 +/- 4.4 IU/l, P < 0.01) and a normal response in 7 patients (delta LH: 7.53 +/- 2.41 IU/l). There was a significant correlation (r = 0.63, P < 0.05) between basal LH levels and LH response to LHRH. Sensitivity and specificity calculated for basal LH levels higher than 6.0 IU/l, considering the LHRH test as reference, were 58% and 85%, respectively. The positive predictive value measuring the possibility of LH higher than 6.0 IU/l to be from a patient with PCO (explosive response to LHRH) was 92%. These data suggest that, in hirsute anovulatory patients, basal LH levels may be a good predictor in the diagnosis of polycystic ovarian syndrome.