Recent progress in the study of the hormonal regulation of spermatogenesis justifies endocrine examination in the case of male sterility. The most important complementary investigation are the assays of FSH, LH and plasmatic testosterone. Radioimmunoassay of FSH is the fundamental examination, since this hormone is considered to be an indicator of germinal function. Thus, in the case of oligospermia, or even azoospermia, FSH assay is decisive. When the FSH levels (in conjunction with LH levels) are high and combined with azoospermia, there is a possibility of testicular dysgenesis linked with a karyotype anomaly (XXY etc.). In some cases the germinal affection is secondary to cryptorchidism, orchitis, torsion, medicinal alteration, or radiotherapy. Decreased testosterone values combined with an insufficient FSH and LH response to stimulation tests indicate a gonadotrophic deficit, which is the best indication for substitution therapy using gonadotrophins or LH-RH. An increase in LH, contrasting with a normal FSH value, evokes the exceptional case of a disturbance of androgen receptivity. Normal FSH (and LH) values suggest excretory sterility. Lastly, when hyperprolactinemia is suspected, an assay of plasmatic prolactin is necessary. A "simple" hormonal evaluation allows a routine etiological approach to the diagnosis of sterility, and is thus an important element in the investigative strategy applied to male sterility, used along with the other complementary and indispensible examinations.