Disorders of Leydig cell function can be primary due to testicular anomalies or secondary to abnormal secretion of LH and FSH. These disorders can be congenital or acquired. The clinical manifestations depend on (1) location of the defect; (2) age at onset of the disorder; and (3) the nature of associated nonreproductive problems. Because of the role of intratesticular testosterone in germ cell maturation, severe Leydig cell dysfunction usually leads to infertility. Diagnosis is based on medical history, physical examination, hormone measurements, including serum testosterone, LH and FSH, and semen analysis. Testosterone replacement therapy is required for androgen deficient males with primary Leydig cell dysfunction. Males with hypogonadotropic hypogonadism may be treated with testosterone to normalize serum testosterone levels, but reversal of infertility requires gonadotropin treatment.