Malaria is still a leading cause of morbidity and mortality. The increase in lipid peroxidation reported in malaria infection and antioxidant status may be a useful marker of oxidative stress during malaria infection. The aim of this study was to investigate the role of antioxidant enzymes against toxic reactive oxygen species in patients infected with Plasmodium vivax and healthy controls. Malondialdehyde levels, superoxide dismutase, and glutathione peroxidase activities were determined in 91 P vivax patients and compared with 52 controls. Malondialdehyde levels, superoxide dismutase, and glutathione peroxidase activities were 8.07 +/- 2.29 nM/ml, 2.69 +/- 0.33 U/ml, and 49.6 +/- 3.2 U/g Hb in the patient group and 2.72 +/- 0.50 nM/ml, 3.71 +/- 0.47 U/ml, and 62.3 +/- 4.3 U/g Hb in the control group, respectively. Malondialdehyde levels were found statistically significant in patients with vivax malaria higher than in healthy controls (P<0.001). On the other hand, superoxide dismutase and glutathione peroxidase activities were found to be significantly lower in vivax malaria patients than in controls (P<0.05). There was an increase in oxidative stress in vivax malaria. The results suggested that antioxidant defense mechanisms may play an important role in the pathogenesis of P vivax.