Acetylcarnitine (AC), present in human spermatozoa and seminal fluid, plays an important role in sperm metabolism. To further investigate the effect of AC on sperm quality, AC (4 g/day) was given to 20 patients with idiopathic oliogasthenospermia for 60 days. AC had no effects on sperm density and total motility, but it did significantly increase progressive sperm motility (mean +/- SEM: 21.7 +/- 3.2% vs 38.2 +/- 4.7). The increment in sperm motility was sustained ( > or = 40%) in 12 patients (mean increment 2.7 fold). This parameter returned to basal value 4 months after therapy discontinuation. Five pregnancies occurred during treatment and only 2 during the 4 months follow-up ensuing therapy discontinuation.