We evaluated the diagnostic significance of single versus repeated human chorionic gonadotropin (hCG) stimulation of testicular steroidogenesis in 25 boys (10 prepubertal group A; 15 early pubertal, group B) with suspected hypogonadism. All subjects received a single injection of hCG (5000 U/1.7 m2) and 1 month later, three repeated injections of 1500 U, one each on alternate days. In 19 out of the 25 boys, testosterone increased normally in both tests: from 20 +/- 6 to 156 +/- 82 ng/dl and from 107 +/- 105 to 615 +/- 293 ng/dl, following a single hCG injection, and from 30 +/- 19 to 439 +/- 298 ng/dl and from 94 +/- 55 to 826 +/- 272 ng/dl, following repeated injections in groups A and B, respectively. The difference between the tests was significant (P < 0.01). Conclusion. Single hCG injection used as a screening test in the evaluation of hypogonadism is conclusive when positive. Only when the initial test is negative may a repeated test help establish the diagnosis.