Purpose: To determine the effects of L-carnitine on sperm parameters in patients with idiopathic sperm abnormalities. Methods: In an academic reproductive care center, 170 patients including 48 smokers and 122 non-smokers participated in a before-after study. Men were given 1 g of L-carnitine orally 3 x daily for 3 months. Sperm assessment was done before and after the treatment. Main outcome measures were sperm concentration, motility, and morphology before and after the treatment. Results: L-Carnitine was effective in improvement of percentile of motile sperms, grade A sperms, and normal-shaped sperms. L-Carnitine significantly improved percentile of motile and grade A sperms in non-smokers. Only the change in the median percentile of normal forms was within the significant range in smokers. Conclusions: The results of this study indicate that smoker patients should not be excluded from the treatment with L-carnitine based on smoking alone.