Purpose: Because varicocele is seen often in infertile men and oxidative stress has been implicated in sperm dysfunction, we assessed spermatozoal reactive oxygen species and seminal total antioxidant capacity in men with and without varicocele. Materials and Methods: Levels of reactive oxygen species and total antioxidant capacity were measured in the semen of 21 infertile men with varicocele, 15 men with incidential varicocele and 17 normal donors without varicocele (controls). Men with leukocytospermia (more than 1 x 106 white blood cells per ml.) were excluded from study. Reactive oxygen species were measured in washed spermatozoa with a luminol dependent chemiluminescence assay. Total seminal antioxidant capacity was measured with an enhanced chemiluminescence assay, and the results were expressed as trolox equivalents. Sperm characteristics were assessed with a computer assisted semen analyzer, and sperm morphology was assessed using World Health Organization and Kruger's strict criteria. Results: Patients with varicocele had significantly higher reactive oxygen species levels than controls (p = 0.02). Reactive oxygen species levels did not differ significantly between infertile and men with incidental varicocele. Total antioxidant levels were significantly lower among men with varicocele (p = 0.02) and those with incidental varicocele compared to controls (p = 0.05). Reactive oxygen species and total antioxidant capacity levels did not correlate in any group. Conclusions: Our results suggest that elevated reactive oxygen species and depressed total antioxidant capacity levels are associated with varicocele. These changes may be related to functional sperm abnormalities and infertility seen commonly in these patients. These findings support a possible rationale for controlled clinical trials of antioxidant supplementation in infertile men with varicocele.