Large quantities of oxygen free radicals can be generated on reperfusion of the ischemic myocardium. This phenomenon is probably responsible for a specific component of cell damage separate from ischemic injury and may blunt the beneficial effects of thrombolysis. Anti-free radical interventions may reduce the severity of this "reperfusion injury". However, reports of treatment failures indicate that reperfusion injury is a complex phenomenon, and suggest that the beneficial effects of treatment may be critically dependent on factors that are not clearly understood.