In a previous paper it was demonstrated that Coenzyme Q10, a lipidic molecule with important antioxidant properties, is present at remarkable levels in human seminal fluid, and shows a direct correlation with seminal parameters (sperm count and motility). In patients with varicocele, on the contrary, correlation with sperm motility was lacking and a higher proportion of Coenzyme Q10 was found in seminal plasma. In the present study, the levels of Coenzyme Q10 in the cell pellet of spermatozoa, obtained after centrifugation of semen, were evaluated. In nonvaricocele subjects it was observed that a higher concentration of Coenzyme Q10 (expressed as ng of the molecule per million of cells) was present in the spermatozoa of oligospermic and asthenospermic patients (sperm count < 20 x 10(6) spermatozoa ml-1, sperm motility < 40%). This relationship was not observed in varicocele subjects, who also showed slightly lower intracellular absolute values of the conenzyme. Since Coenzyme Q10 is an antioxidant molecule involved in the defence of the cell from free radical damage, higher intracellular concentrations may represent a mechanism of protection of the spermatozoa. In varicocele patients, this mechanism could be deficient, leading to higher sensitivity to oxidative damage.