Background: Male infertility is responsible for 35% of infertile couples. Aim: To investigate the causes of male infertility and the relative importance of endocrine factors. Patients and methods: Patients referred to an andrology clinic due to an abnormal spermiogram were studied. A testitular examination, spermiogram and determination of FSH, LH, testosterone and prolactin were done to all. Testicular biopsy was done to patients with severe oligospermia or azoospermia. Causes of infertility were defined and classified as pretesticular, testicular, posttesticular or unclassified. Results: Two hundred fifty seven males were studied. In 3.5% of them, the cause of infertility was defined as pretesticular (that included hypothalamic and pituitary endocrine causes), in 66.9% it was classified as testicular, in 15.6% as posttesticular and in 14%, as unclassified. Thirty percent of infertility cases were idiopathic, 17.9% were associated to varicocele, 12.8% were associated to cryptorchidism, 8.9% to Klinefelter syndrome and 6.6% to exposure to toxic substances. In 50% of patients with cryptorchidism, this abnormality was found during the specialized andrological examination and referrals for surgical correction were made late. Two thirds of patients with Klinefelter syndrome were hypoandrogenic. Conclusions: Causes for male infertility should be investigated and diagnosed accurately. Primary hypoandrogenic testicular failures must be treated with hormone replacement therapy