Aromatase inhibition by delta 1-testolactone (Teslac, 500 mg twice daily) for 6 months in 9 patients with idiopathic oligozoospermia lowered the levels of serum estradiol (E2) and thereby sex hormone binding globulin (SHBG) (rS = +0.40, p less than 0.025) to values -35 and -25%, respectively, below the pretreatment values (P less than 0.001 and less than 0.005). The E2 decrease was accompanied by a temporary increase (+50%) in the levels of follicle stimulating hormone (FSH), not of luteinizing hormone (LH), and of 17 alpha-hydroxyprogesterone (17 alpha-OHP), but less of testosterone (T) (+30%), which led to a transient rise in the 17 alpha-OHP/T ratio. The T/E2 ratio and "free T" index (T/SHBG) almost doubled until the end of the treatment period. During delta 1-testolactone treatment the mean sperm density gradually rose from 8.1 +/- 1.3 (SEM) before to 21.3 +/- 6.7 X 10(6)/ml after 6 months (P less than 0.01), whereas the total sperm count almost threefold increased (P less than 0.05). Sperm concentrations exceeding 20 X 10(6)/ml were achieved in 4 of the 9 patients. Two of these patients' wives became pregnant. Although the data point to a pivotal role of estrogens in the pathogenesis of the spermatogenic lesion in some patients with idiopathic oligozoospermia, the lack of a beneficial effect of estrogen lowering in others points to a multicausal nature of the disease entity.