A multicentre study (ten centres) in seven countries was done to assess the contraceptive efficacy of hormonally-induced azoospermia in 271 healthy fertile men. Each subject received 200 mg testosterone enanthate weekly by intramuscular injection. 157 men (cumulative rate at 6 months 65%) became azoospermic in three consecutive semen samples. These men entered a 12-month efficacy phase during which continuing testosterone injections were the only form of contraception. There was 1 pregnancy during 1486 months of the efficacy phase (0·8 conceptions [95% confidence interval 0·02-4·5] per 100 person- years). Discontinuations from the study were mainly because azoospermia was not achieved within 6 months and because of dislike of the injection schedule. The mean time to become azoospermic was 120 days (SD 40); reappearance of spermatozoa was detected in 11 men and in no case led to discontinuation from the study or to pregnancy. After the testosterone injections had been stopped, the estimated median time from azoospermia to recovery (sperm concentration of at least 20 million/ml) was 3·7 months (3·6-3·9) and to the subject's mean baseline sperm concentration was 6·7 months (6·2-8·7). Hormonal regimens that induce azoospermia can provide highly effective, sustained, and reversible male contraception with minimum side-effects.