The increase of L-carnitine and acetyl-L-carnitine content in the spermatozoa during the epididymal passage is associated with acquisition of progressive motility. Previous studies have shown that seminal free L-carnitine correlates with sperm count and motility, a reduction of acetyl-L-carnitine/carnitine ratio has been found in patients with defective sperm motility. The aim of this work was to evaluate any possible effect of L-carnitine on spermatozoal motility in a group of patients with unexplained idiopathic asthenozoospermia. Patients were randomized into 2 groups to start with L-carnitine (2 grams/day) orally in 3 divided doses after meals for 3 months or with a seemingly identical placebo. After 3 months, a cross over was done between the 2 groups. The first hour motility increased in the carnitine group than the placebo group that was statistically significant. No change in other semen parameters could be detected. No side effects were reported. In conclusion, the use of L-carnitine for idiopathic athenozoospermia may improve the percentages of actively motile sperms.