The aim of the study described here was to evaluate any possible effect of L-carnitine on spermatozoal motility in a group of patients with unexplained asthenozoospermia in four different infertility centres. One hundred patients received 3 g d-1 of oral L-carnitine for 4 months. Sperm parameters were studied before, during and after this treatment. Motility was also studied by means of a computer-assisted sperm analysis. The results of the study indicate that L-carnitine is able to increase spermatozoal motility, both in a quantitative and in a qualitative manner (per cent motile spermatozoa increased from 26.9 +/- 1.1% to 37.7 +/- 1.1% [P < 0.001]; per cent spermatozoa with rapid linear progression increased from 10.8 +/- 0.6% to 18.0 +/- 0.9% [P < 0.001]; mean velocity increased from 28.4 +/- 0.6 microns s-1 to 32.5 +/- 0.8 microns s-1 [P < 0.001]; linearity index increased from 3.7 +/- 0.1 to 4.1 +/- 0.1 [P < 0.001], especially in the subgroup of patients with poor rapid linear progression of spermatozoa (per cent of motile spermatozoa increased from 19.3 +/- 1.9% to 40.9 +/- 1.4% [P < 0.001], and per cent of spermatozoa with rapid linear progression increased from 3.1 +/- 0.4% to 20.3 +/- 1.6% [P < 0.001]) An increase in spermatozoal output was also observed (total number of ejaculated spermatozoa increased from 142.4 +/- 10.3 10(6) to 163.3 +/- 11.0 x 10(6) [P < 0.001]). The authors conclude that oral administration of L-carnitine may improve sperm quality at least in patients with idiopathic asthenozoospermia.