Objective: To evaluate the degree of E-2 deficiency in male congenital hypogonadotropic hypogonadism (CHH), and its response to different hormonal treatments.Design: Retrospective and prospective studies.Setting: Academic institution.Patient(s): Untreated or treated CHH, healthy men, untreated men with Klinefelter syndrome (KS). Intervention(s): Serum sex hormone-binding globulin (SHBG) and total E-2 (TE2) as well as bioavailable (BE2) and free (FE2) levels were measured and determined.Main Outcome Measure(s): Total, bioavailable, and free testosterone, TE2, BE2, FE2 were compared in normal men, untreated and treated CHH and in untreated KS.Result(s): TE2, BE2, and FE2 levels were very significantly lower in untreated patients with CHH (n = 91) than in controls (n = 63) and in patients with KS (n = 45). The TE2 correlated positively with serum total T in patients with CHH. The TE2 also correlated very positively with serum LH in the combined population of patients with CHH and healthy men, suggesting that low E-2 levels in CHH are due to severe LH-driven T deficiency. All fractions of circulating E-2 were very significantly higher in patients with CHH receiving T enanthate (n = 101) or the FSH-hCG combination (n = 88) than in untreated patients with CHH. Contrary to dihydrotestosterone (DHT), both T enanthate and combined FSH-hCGtherapy significantly and prospectively increased TE2 levels in patients with CHH.Conclusion(s): Contrary to KS, the male hypogonadism observed in CHH is associated with profound E-2 deficiency, which can be overcome by aromatizable androgen or combined gonadotropin therapy. (Fertil Steril (R) 2011; 95: 2324-29. (C) 2011 by American Society for Reproductive Medicine.)