Objective To assess the effectiveness of testosterone undecanoate on sperm motility and pregnancy incidence in men with asthenospermia.Methods A clinical trial was performed.Fifty men with asthenospermia were included to receive placebo (control group,n=9) or T undecanoate 80 mg/d (study group,n=41).Pregnancy incidence and sperm characteristics after 1,2 and 3 months of medication and 3 months after the end of the trial were measured.Results Compared with the placebo,T undecanoate treatment produced a satisfactory improvement of seminal motility (F=55.904,P=0.000).In study group,the incidence of pregnancy was 28.2％ while the incidence of pregnancy in control group was 11.1％.In study group,26 patients took T undecanoate for more than 3 months.In the 3 months,semen volume showed no statistical difference (F=1.206,P=0.312) before and after treatment,sperm concentration (F=0.023,P=0.000) and motility a,b,a+b (P=0.000)showed statistical differences.Motility grade a showed significantly higher increment in 2 and 3 months after treatment than 1 month and there was no statistical difference between 2 and 3 months.So did grade b.Conclusion The results indicate that T undecanoate increases seminal motility,leading to a higher incidence of pregnancy in couples with infertility related to asthenoaspermia.