To determine the function of Leydig cells in patients with varicocele, hCG-stimulated levels of progesterone (Prog), 17 alpha-Hydroxy-4-pregnene-3,20-dione (17OHP), 4-Androstene-3,17-dione (A-dione), testosterone (T), and estradiol-17 beta (E2) in both spermatic and peripheral veins were measured. Seventy-two patients with idiopathic varicocele were divided into four groups: patients in group 1 were untreated, whereas patients in groups 2, 3, and 4 were given a single i.m. injection of 10,000 IU hCG 24 h, 96 h, and 168 h before surgery, respectively. In the spermatic and peripheral veins, levels of Prog, 17OHP, and E2 showed peaks at 24 h, whereas levels of A-dione and T showed peaks at 96 h. Significant increases in the ratios of spermatic veins 17OHP to A-dione and 17OHP to T, and a significant decrease in the ratio of T to E2, was found 24 h following hCG treatment. These results demonstrate that, following hCG injection, there is a transient inhibition of testicular C17-20-lyase activity, probably mediated by E2, even in subfertile males with varicocele.