An LH-RH test was performed before and 44 and 92 h after treatment with 2.5 mg oestradiol benzoate in 17 patients with a diagnosis of polycystic ovarian (PCO) disease. The responses were compared with the same tests performed on ten normal subjects during the early follicular phase of their menstrual cycles (days 4--6). Patients were divided into two groups on the basis of their LH responses to LH-RH. In Group A (seven patients) the response at 92 h was greater than at 44 h as in the normal subjects, but in Group B (ten patients) the response at 44 h was greater than at 92 h. Basal serum hormone values were similar in the two groups except for androgens and oestrone, which were significantly (P less than 0.01) higher in Group B patients. There was a negative correlation between the basal androgen and oestrone concentrations and the LH and FSH amplifications at 92 h in all PCO patients. The ratios of the basal concentrations of LH to FSH and the ratios of the highest levels of each achieved during the basal LH-RH test, were significantly higher in the two groups of patients when compared to controls (P less than 0.01). The test is of value in predicting the subsequent responsiveness to clomiphene. All patients in Group A showed evidence of ovulation following treatment with 100 mg clomiphene for 5 days, but only one of Group B responded in this way.