An association between prostatitis and male infertility has been suspected, yet is poorly understood. Prostatitis is often associated with granulocytes in the prostatic fluid that generate reactive oxygen species (ROS), known to impair male fertility. We compared ROS, the total antioxidant capacity (TAC), and a novel index of oxidative stress (ROS-TAC score) in patients with chronic prostatitis and in healthy controls. Semen specimens from 36 men with chronic prostatitis (National Institutes of Health category IIIa), 8 men with prostatodynia (National Institutes of Health category IIIb), and 19 controls attending our urologic clinic were examined according to the World Health Organization criteria. Leukocytospermia was measured by the Endtz test (myeloperoxidase assay). ROS and TAC production was measured by chemiluminescence assay. A composite ROS-TAC score was also calculated in patients and controls. The sperm concentration, percentage of motility, and morphology among the groups did not differ. The mean +/- standard error log-transformed ROS level was significantly higher in patients with leukocytospermia (3.2 +/- 0.6) than in patients without leukocytospermia (1.8 +/- 0.2; P = 0.04) and controls (1.3 +/- 0.3, P = 0.01). TAC was significantly lower in patients with or without leukocytospermia (859.69 +/- 193.0 and 914.9 +/- 65.2, respectively) than in controls (1653.98 +/- 93.6, P = 0.001). The mean ROS-TAC score of controls (50.0 +/- 4.1) was significantly higher than those of patients with chronic prostatitis and leukocytospermia (8.2 +/- 9.2) and those without leukocytospermia (34.2 +/- 2.9; P <0.001). Men with chronic prostatitis or prostatodynia have seminal oxidative stress, irrespective of their leukocytospermia status. These observations may help shed light on the long-standing controversy surrounding prostatitis and infertility.