The purpose of the current study was to investigate a novel measure of oxidative stress in patients with acute coronary syndrome (ACS) using a pro-oxidant-antioxidant balance (PAB) assay, which is simple and rapid. Blood samples were taken from 94 patients admitted with ACS. Pro-oxidant-antioxidant balance values were determined in the first and second 12 hours after the onset of symptoms and compared with values for 81 age- and sex-matched controls. The mean PAB values in the first and second 12-hour samples of patients were both significantly higher than that of controls (P < .001). Among patients, PAB values were also significantly higher in the second samples compared with the first samples (P < .001). These findings indicate a heightened state of oxidative stress following ACS and suggest that the PAB value may be considered as a cardiovascular risk predictor to estimate the extent of oxidative stress.