The purpose of this study was to assess the incidence, features, and pathogenesis of microscopic injuries to the cardiac conduction system caused by cardiopulmonary resuscitation. : Prospective study. Autopsy unit of the university. Victims who had been transferred to the emergency room due to cardiac arrest arising from nontraumatic cause plus age-matched control patients. None. The cardiac conduction system of 80 hearts without gross injury from patients who had received cardiopulmonary resuscitation due to nontraumatic causes was examined. Of these 80 patients, seven (9%) showed fresh injuries, including a lesion that had gone unreported in the previous literature. Localized hemorrhage without inflammatory reaction was evident in six of these patients. Three of the six patients showed hemorrhage in the sinoatrial node, whereas the other three patients showed hemorrhage in the atrioventricular conduction system. The remaining one patient showed localized dissection of the atrioventricular node artery with the appearance of red blood cells in the false lumen. There was no significant difference with regard to age, gender, cause of cardiopulmonary arrest, whether victim had received electrical shock treatment, whether victim had received anticoagulants, and the duration of cardiopulmonary resuscitation between the seven patients with fresh injuries and the other 73 patients. Fracture of the sternum or rib was found in only one of the seven patients but in 14 of the 73 patients. No pathologic lesions were found in the 30 control patients who did not receive cardiopulmonary resuscitation. It can be presumed that injuries to the conduction system do occur in limited regions during cardiopulmonary resuscitation. Minute differences in the location of the cardiac silhouette or cardiac conduction system also need to be considered, rather than just the severity of force to the anterior chest, when determining the pathogenesis of these injuries.