Some infertile males with nonobstructive oligo- or azoo-spermia have an endocrinopathy with the serum testosterone that is below normal or at the low end of normal, while the serum estradiol is elevated or at the high end of normal. The ratio of testosterone-to-estradiol in these patients is decreased. If these patients are treated with the aromatase inhibitor testolactone, then the sperm count may increase enough to allow the patient to be fertile.