Generation of oxygen free radicals at the time of reperfusion has been implicated as a major causative factor for myocardial reperfusion injury. In order to examine if oxygen free radical scavenger superoxide dismutase (SOD) in human plasma prevent reperfusion injury with successful thrombolysis and limit infarct size, we studied the relationship between infarct size and plasma SOD activity in 25 patients received thrombolytic therapy in antero-septal acute myocardial infarction. Plasma SOD activities were measured by the nitrate method. There was no variance in plasma SOD activity in patients with acute myocardial infarction before and after thrombolysis. Infarct size was evaluated by using % abnormally contracting segments (% ACS) calculated with left ventriculography at chronic stage. Humanplasma SOD activities had a significant negative correlation with % ACS (r= -0.58, p <0.05) in 17 patients with successful thrombolysis. This significant correlation was still shown in 14 patients without collaterals before recanalization (r= -0.56, p <0.05). On the other hand, there was no significant relationship between plasma SOD activities and % ACS in 8 patients with a complete occlusion after thrombolysis (r= -0.081). These results showed that human plasma SOD prevent the reperfusion injury and limit the infarct size in patients with successful thrombolysis and imply that the generation of free radical at the time of reperfusion results in myocardial reperfusion injury.