OBJECTIVE: To determine the efficacy of combined l-carnitine and l-acetyl-carnitine therapy in infertile males with oligo-astheno-teratozoospermia. DESIGN: Placebo-controlled double-blind randomized trial. SETTING: University tertiary referral center. PATIENT(S): Sixty infertile patients (aged 20-40 years) with the following baseline sperm selection criteria: concentration, 10 to 40 x 10(6)/mL; forward motility, <15%; total motility, 10% to 40%; and atypical forms, <80%. Fifty-six patients completed the study. INTERVENTION(S): Patients were submitted to a combined treatment of l-carnitine (2 g/d) and l-acetyl-carnitine (1 g/d) or of placebo; the study design was 2 months' wash-out, 6 months of therapy or of placebo, and 2 months' follow-up. MAIN OUTCOME MEASURE(S): Variation in the semen parameters that were used for patient selection. RESULT(S): Even though increases were seen in all sperm parameters after combined carnitine treatment, the most significant improvement in sperm motility (both forward and total) was present in patients who had lower initial absolute values of motile sperm (<4 x 10(6) forward or