A 28-year-old man with adult-onset idiopathic male hypogonadotropic hypogonadism (MHH) is reported. He had been delivered normally and had normal puberty. He was referred to our hospital with a chief complaint of infertility. Serum levels of testosterone, luteinizing hormone, and follicle stimulating hormone (FSH) were low. Semen analysis demonstrated azoospermia. Pituitary hypofunction was suggested by gonadotropin releasing hormone (GnRH) loading test. Magnetic resonance images did not detect any abnormalities in the hypothalamic-pituitary region. After a diagnosis of adult-onset hypogonadotropic hypogonadism was established, the patient received human chorionic gonadotropin (hCG) and recombinant FSH treatment. After 5 months, his sperm count reached 6.9 x 10(6) per ml and his wife became pregnant. Adult-onset HH in most cases is caused by tumors and trauma. To our knowledge 17 cases of adult-onset idiopathic HH have been reported, and there were only 3 cases that were caused by pituitary dysfunction. This report showed that r-FSH and hCG therapy was effective in promoting fertility in a patient with adult-onset idiopathic MHH.