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Context: Previous male contraceptive studies showed that progestins enhance spermatogenesis suppression by androgens in men., Objective: We compared the efficacy of spermatogenesis suppression by the combination of levonorgestrel (LNG) with testosterone (T) implants to that by T implants alone in two different ethnic groups., Design: This was a randomized trial performed in two centers with two treatment groups., Settings: The study was performed at the Academic Medical Center in the United States and the Research Institute in China., Participants: Forty non-Chinese and 40 Chinese healthy male volunteers were studied., Interventions: Subjects were randomized to receive four LNG implants together with four T implants (inserted on d 1 and wk 15-18) vs. T implants alone for 30 wk., Main Outcome Measures: The primary end point compared the efficiency of suppression to severe oligozoospermia (1 x 10(6)/ml) by LNG plus T implants vs. that by T implants alone. The secondary end point examined differences in spermatogenesis suppression between Chinese and non-Chinese subjects., Results: LNG plus T implants caused more suppression of spermatogenesis to severe oligozoospermia during the treatment period than T implants alone at both sites (P < 0.02). In Chinese men, severe oligozoospermia was achieved in more than 90% of the men in both treatment groups. Suppression to severe oligozoospermia was less in the non-Chinese men (59%) after T alone (P < 0.020); this difference disappeared with combined treatment (89%). T implant extrusion occurred in six men. Acne and increased hemoglobin were the most common adverse events., Conclusion: T implants resulted in more pronounced spermatogenesis suppression in Chinese men. Addition of LNG implants to T implants enhanced the suppression of spermatogenesis in the treatment period in both Chinese and non-Chinese men.