This is a pilot dose-finding study of spermatogenic suppression using testosterone undecanoate (TU) injections alone in normal Chi- nese men. Thirty-two healthy men were recruited. Volunteers un- derwent pretreatment evaluation, then a treatment period in which group I (n 5 13) received 500 mg TU, group II (n 5 12) received 1000 mg TU, and group III (n 5 7) received placebo, respectively, at monthly intervals during the treatment period (or until azoospermia was achieved). Thereafter, they underwent a recovery period until all parameters returned to pretreatment levels. Eleven of 12 volunteers in the 500-mg TU group, and all volunteers in the 1000-mg TU group became azoospermic. Faster suppression of spermatogenesis was achieved in the 1000-mg TU group. Serum testosterone increased significantly in the higher dose group at weeks 8 and 12, but remained within the normal range. Mean serum LH and FSH were profoundly suppressed by both doses to undetectable levels at week 16. TU in- jections did not cause a significant change in high density lipoprotein cholesterol levels. No serious side-effects were found. We conclude that both dosages of TU can effectively, safely, and reversibly sup- press spermatogenesis in normal Chinese men. (J Clin Endocrinol Metab 84: 3642-3647, 1999)