Gonadotropin response to exogenous luteinizing hormone-releasing hormone (LHRH) was studied in two groups of patients with the polycystic ovary syndrome (PCOS). Group I (n = 44) was diagnosed as 'overt PCOS' based on clinical and endocrine abnormalities, and the typical ultrasonic picture of multicystic changes in an increased amount of ovarian stroma. Group II patients (n = 34), with similar clinical and hormonal changes, were classified as 'borderline PCOS' because endosonography of the ovaries was not conclusive. Serum gonadotropins were followed 15, 30, 45 and 60 min after administration of 100 micrograms LHRH intravenously. Plasma LH and the peak ratio of luteinizing hormone to follicle stimulating hormone (LH/FSH peak) after LHRH were significantly higher in Group I and II patients (p less than 0.001) than in controls (n = 11). There was a significant positive correlation between LH (r = 0.73 and 0.68, p less than 0.05) and LH/FSH ratio peaks (r = 0.61 and 0.68, p less than 0.05) after LHRH, and the basal values in each group. However, hyperreactivity of LH and the LH/FSH ratio after LHRH (defined as values exceeding the 95th percentile of control values) was only present in Group I in 38.6 and 56.8%, and in Group II in 47.1 and 73.5% of patients. It is concluded that: exaggerated LH release after LHRH is a typical yet not unique feature of PCOS and its sensitivity is inferior to its specificity; a higher rate for sensitivity is achieved when the LH/FSH ratio after LHRH (instead of LH release alone) is used; and that hyperreactivity of LH after LHRH is neither correlated with the plasma concentrations of total testosterone, free plasma testosterone or androstenedione, nor with the ultrasonic picture of the ovaries as significantly higher (p less than 0.05) LH and LH/FSH ratio peaks were found in borderline PCOS patients. While the heterogeneity of gonadotropin response to LHRH clearly limits its routine diagnostic use, the FSH and LH response is useful in patients with inconsistent clinical and ultrasonic features suggestive of PCOS.