Congestive heart failure (CHF) occurs in about one half of all patients with acute myocardial infarction and is a manifestation of acute alterations in left ventricular function. In the present study CHF is defined on clinical grounds, according to the presence and extent of bilateral pulmonary rales. An accompanying S3ventricular gallop was heard in 58% of our patients with heart failure initially, but it disappeared eventually in the majority. Dilatation of pulmonary veins and blurring of pulmonary vascular markings are useful roentgenographic signs which reflect elevations in left heart filling pressure. At times the earliest indicators of heart failure, these findings appear in general to be less sensitive than the physical examination in diagnosing CHF. Although stroke volume is decreased with CHF, cardiac index is generally maintained by increased heart rate. Left ventricular minute work and stroke work are significantly decreased, while left ventricular end-diastolic pressure is significantly increased, in patients with CHF complicating acute myocardial infarction. Arterial hypoxemia is common and the degree of arteriovenous shunting is roughly proportional to the elevation of left ventricular filling pressure. The mortality of patients with CHF is approximately three times that of patients with acute myocardial infarction and no complications. Diuretic therapy is safe and effective. Attention is called to the probability that the use of digitalis preparations in the early hours following myocardial infarction is hazardous. Furthermore, hemodynamic benefit from digitalization in the early postinfarction period remains unproven.