In clinical practice, the human chorionic gonadotrophin (hCG) stimulation test is widely used to evaluate testicular function. Inhibin B, a gonadal peptide regulating follice-stimulating hormone (FSH) secretion, is an established marker of Sertoli cell function and spermatogenesis in adults. The aim of this study was to determine whether basal inhibin B levels are able to predict testosterone response to hCG in idiopathic hypogonadotropic hypogonadism (IHH) patients and to evaluate the correlation between inhibin B and gonadotropins in these patients and controls. Inhibin B (n=15) and other hormones (n=29) were measured in 29 patients with IHH and 32 controls. Inhibin B (n=8) and testosterone levels (n=25) before and after hCG stimulation were measured in 25 male patients with IHH by an immunoassay specific for inhibin B. Basal inhibin B was compared to the testosterone increase after hCG. There was a significant increase in inhibin B (22.6 +/- 9.8 vs 45.07 +/- 13 pg/mL; p=.005), free testosterone (2.92 +/- 0.55 vs. 7.9 +/- 1.5 pg/mL; p=.002), and total testosterone (69.0 +/- 15.9 vs. 184.9 +/- 44.1 ng/mL; p = .013) levels 72 hours after hCG injection. Inhibin B and the hCG-induced free testosterone and total testosterone increment correlated strongly (r=0.802, P<.001; r=0.793, P<.001, respectively). We conclude that basal inhibin B predicts the testosterone response to hCG in IHH patients and therefore gives reliable information about Leydig cell reserve. Furthermore, inhibin B levels show negative correlation with luteinizing hormone (LH) in control patients and positive correlation with FSH and LH in IHH patients. LH may effect inhibin B secretion. Further studies are necessary to define the physiology of inhibin B in human males.