Toxoplasma gondii, a common parasitic infection, has a special affinity to the brain. It has a life-long existence without an apparent clinical disease. While the etiology of Bipolar disorder (BD) remains unclear, epidemiological studies suggest a role for infections. CNS is particularly susceptible to oxidative stress because of its high metabolic rate and its low levels of antioxidant defenses. Oxidative stress is a contributor to the initiation and progression of many neurological illnesses. Oxidative stress injury is a constantly and compelling finding associated with BD and toxoplasmosis. This cross-sectional study has investigated a possible role of toxoplasma-induced oxidative stress in the development of BD. Healthy controls and BD patients were examined for anti ToxoplasmaIgG and two; protein (3-nitrotyrosine) and DNA (8-hydroxy-2′-deoxyguanosine) oxidative stress markers. Toxoplasmapositivity was higher (40%) among BD patients compared to controls (12%). Significantly higher levels of anti-ToxoplasmaIgG were detected in BD patients compared to controls. Nitrotyrosine (796.7±106.28) and especially 8-OHdG (20.31±8.38) were significantly higher, among Toxo-positive BD compared to Toxo-negative BD (675.97±144.19; 7.44±2.86) and healthy controls (464.02±134.6; 4.17±1.43). These findings might indicate a role for Toxoplasmainfection in the development of BD possibly through creating a highly oxidative brain environment.