Context: The development of a safe and effective reversible method of male contraception is still an unmet need. Objective: Evaluation of suppression of spermatogenesis and contraceptive protection by coadministered im injections of progestogen and testosterone. Design: Prospective multicentre study. Setting: Ten study centers. Participants: Healthy men, aged 18-45 years, and their 18- to 38-year-old female partners, both without known fertility problems. Intervention: Intramuscular injections of 200-mg norethisterone enanthate combined with 1000-mg testosterone undecanoate, administered every 8 weeks. Main Outcomes Measures: Suppression of spermatogenesis by ejaculate analysis, contraceptive protection by pregnancy rate. Results: Of the 320 participants, 95.9 of 100 continuing users (95% confidence interval [CI], 92.8- 97.9) suppressed to a sperm concentration less than or equal to 1 million/mL within 24 weeks (Kaplan-Meier method). During the efficacy phase of up to 56 weeks, 4 pregnancies occurred among the partners of the 266 male participants, with the rate of 1.57 per 100 continuing users (95% CI, 0.59-4.14). The cumulative reversibility of suppression of spermatogenesis after 52 weeks of recoverywas94.8 per 100 continuing users(95%CI, 91.5-97.1).Themostcommonadverse events were acne, injection site pain, increased libido, and mood disorders. Following the recommendation of an external safety review committee the recruitment and hormone injections were terminated early. Conclusions: The study regimen led to near-complete and reversible suppression of spermatogenesis. The contraceptive efficacy was relatively good compared with other reversible methods available for men. The frequencies of mild to moderate mood disorders were relatively high.