To determine contraceptive efficacy of hormonaly induced sperm suppression to severe oligozoospermia or azoospermia. Prospective, non-comparative contraceptive efficacy study. Multicenter study in 15 centers in nine countries. Three hundred ninety-nine normal, healthy, fertile men requesting a male contraceptive method. Weekly intramuscular injection of 200 mg T enanthate. Incidence of pregnancies and efficacy when couples relied on T injections alone for contraception. Four pregnancies occurred during 49.5 person-years involving men with oligozoospermia (0.1-3 x 10 6/ml) and none during 230.4 person-years in azoospermic men: pregnancy rates 8.1 (95% CI 2.2-20.7) and 0.0 (95% CI 0.0-1.6) per 100 person-years, respectively, or 1.4 (95% CI 0.4-3.7) per 100 person-years for oligozoospermia and azoospermia (0-3 x 10 6/ml) combined. Pregnancy rates were related to sperm concentration. Inadequate suppression of spermatogenesis occurred in eight men and escape from suppression occurred in four. Discontinuations were due to personal reasons (50 men, cumulative annual life-table rate 12.2%[95% CI 9.1-16.1%]) and dislike of the injection schedule (21 men, 5.1%[95% CI 3.2-7.9%]). Thirty-five men discontinued for medical reasons (9.4%[95% CI 6.7-13.2%]), with no serious treatment-related side effects. After stopping injections, sperm output recovered; additionally, fertility was demonstrated in 33 couples. Suppression of spermatogenesis to azoospermia or severe oligozoospermia (3 x 10 6/ml) induced by weekly T enanthate injections results in sustained, reversible contraception with good efficacy and minimal short-term side effects. New hormonal regimens with more convenient delivery and improved spermatogenic suppression would provide practical male contraception.