The results of treatment of 250 patients with established acute myocardial infarction in a coronary care unit in a university hospital are described. The criteria for diagnosis have been carefully defined. In 62 percent of patients admitted with a tentative diagnosis of acute infarction, the initial impression was confirmed. Fifteen percent of patients admitted to the unit were classified as having possible infarction; in this group, the mortality rate was 3 percent. A classification of functional severity based on clinical evidence of heart failure or shock is presented. Morbidity and mortality in acute myocardial infarction are related to the functional severity of the illness. Although arrhythmia is common, the overriding importance of five life-threatening arrhythmias is emphasized. Mortality of patients in the coronary care unit was not improved in comparison to those treated under regular care until strong central direction of therapeutic programs, immediate treatment of arrhythmia in cardiac arrest, and delegation of some medical authority to trained nurses was accomplished. The change in concept of the purposes and practices of special coronary care from resuscitation to prevention of arrhythmia is emphasized. The mortality in myocardial infarction complicated by shock remains high. In the absence of shock, aggressive medical treatment in the coronary care unit reduced mortality from 26 to 7 percent. The implications of these data in the management of patients admitted to a hospital with a diagnosis of acute myocardial infarction are discussed.