This report describes a Phase II, multicenter, contraceptive efficacy clinical trial using monthly injections of testosterone undecanoate (TU) alone at a dose of 500 mg in healthy Chinese men. Three hundred eight healthy men were recruited in six centers distributed throughout China. Volunteers underwent a control period with no treatment, then a 12-month treatment period including a 6-month suppression phase followed by a 6-month efficacy phase and a 12-month recovery period. During the suppression phase, an initial loading dose of 1000 mg TU, followed by 500 TU at monthly intervals were given until azoospermia or severe oligozoospermia was achieved, up to a maximum of six injections. During the efficacy phase, 500 mg TU were administered at monthly intervals for 6 months. Nine of 308 men did not achieve azoospermia or severe oligozoospermia (<3 x 10(6)/ml) within the 6-month suppression phase. This gave a methodological failure rate of 2.9/100 couple years (95% confidence interval of 1.0-4.8/100 couple years). Two hundred ninety-six men entered the efficacy phase. The continuation rate during the efficacy phase was 95/100 couple years. There were no pregnancies caused by men who achieved azoospermia or severe oligozoospermia. Reappearance of sperm occurred in six men during the efficacy phase, and one pregnancy was attributed to sperm rebound. This gave a secondary failure rate of 2.3/100 couple years (95% confidence interval of 0.5-4.2/100 couple years). Thus, the total failure rate was 5.2%, and total efficacy was 94.8%. Spermatogenesis in all subjects returned to the normal reference range within the recovery period. The mean serum testosterone concentration increased 131%, and the mean serum LH and FSH concentrations decreased 72% and 70%, respectively, after TU injections during the treatment period. The mean level of serum high density lipoprotein cholesterol decreased (14%), and the mean hematocrit increased 6% compared with baseline. No serious adverse events and no significant changes in serum chemistry occurred during the study. The results showed that monthly TU injection at a dose of 500 mg after an initial loading dose of 1000 mg can effectively, safely, and reversibly suppress spermatogenesis in healthy Chinese men without serious adverse effects.