Objectives To assess the effectiveness and safety of carnitine in the treatment of idiopathic asthenozoospermia. Methods The Cochrane Library, MEDLINE, EMbase, and CNKI were searched between Jan 1995 and Dec 2006. Both English and Chinese studies were included in the review if they were randomized controlled trials (RCTs) involving men with idopathic asthenozoospermia who were treated with carnitine. Trial screening, data extraction, and quality assessment of included trials were conducted by method recommended by Cochrane Collaboration. Statistical analysis was conducted using RevMan 4.2.10 software. Results Five RCTs involving 346 patients met the inclusion criteria, and 307 patients were included in the meta-analysis. The results showed that: after being treated with carnitine for 3 and 6 months, the difference of the patients’ partners’ spontaneous pregnancy rate between treatment group and control group was statistically significant with RR2.46 and 95% CI1.12 to 5.43 (Z=2.23, P=0.03). After being treated with carnitine for 3 and 6 months, the difference of forward motile sperm per ejaculate between treatment group and control group was not statistically significant with WMD 9.16 and 95%CI 0.14 to 18.18 (Z=1.99, P=0.05) and WMD 5.28 and 95%CI –4.45 to 15.01 (Z=1.06, P=0.29). After being treated with carnitine for 3 and 6 months, the difference of percentage of forward sperm motility between treatment group and control group was not statistically significant with WMD 14.56 and 95%CI –4.49 to 33.61( Z=1.50 ,P=0.13), and WMD 7.34 and 95%CI –5.93 to 20.61 (Z=1.08, P=0.28). After being treated with carnitine for 3 and 6 months, the difference of total motile sperm per ejaculate between treatment group and control group was not statistically significant with WMD 15.32 and 95%CI –1.34 to 31.98 (Z=1.80, P=0.07) and WMD 6.20, 95%CI –3.00 to 15.39 (Z=1.32, P=0.19).After being treated with carnitine for 3 and 6 months, the difference of percentage of total sperm motility between treatment group and control group was not statistically significant with WMD 2.97 and 95%CI –5.75 to 11.69 (Z=0.67, P=0.50) and WMD 4.48 and 95%CI-9.17 to18.14 (Z=0.64, P=0.52). After being treated with carnitine for 3 and 6 months, the difference of semen volume between treatment group and control group was not statistically significant with WMD –0.12 and 95%CI –0.55 to 0.30 (Z=0.57, P=0.57) and WMD 0.03 and 95%CI –0.38 to 0.45 (Z=0.16, P=0.87). After being treated with carnitine for 3 and 6 months, the difference of sperm concentration between treatment group and control group was not statistically significant with WMD 7.92 and 95%CI – 2.85 to18.68 (Z=1.44, P=0.15), and WMD 1.02 and 95%CI –5.09 to 7.14 (Z=0.33, P=0.74). Three RCTs reported that there were no serious side effects of carnitine during the treatment period. Conclusions The available evidence indicates that spontaneous pregnancy rate would increase with carnitine therapy, while it is short of improvement of semen parameters. There is no serious side effect of carnitine. Because of lack of evidence, we cannot conclude that carnitine is effective in improving the prognosis of infertile patients with idiopathic asthenozoospermia. More high quality trials with large sample are proposed.