A 23-year-old man visited our hospital, complaining of a lack of secondary sexual characteristics. Based on both clinical and endocrinological examinations, he was diagnosed as suffering from idiopathic hypogonadotropic hypogonadism (IHH). With human chorionic gonadotropin (hCG)/human menopausal gonadotropin (hMG) replacement therapy, clinical symptoms and serum testosterone levels improved to normal ranges. Interestingly, prostate volume as measured by transrectal ultrasonography increased considerably from 3.6 ml before treatment to 20.2 ml after treatment. The monitoring of prostate volume might be a parameter useful for evaluating the treatment effect of hCG/hMG replacement therapy in IHH.