Serial measurements of arterial blood lactate were compared with established hemodynamic, respiratory, and metabolic measurements as prognostic indicators of survival or fatality in 28 patients with acute myocardial infarction complicated by heart failure and/or cardiogenic shock. Measurements were compared at the time of admission, at the time of maximal response to therapy, and 4 h prior to discharge or the onset of the agonal period. Blood lactate was the only measurement which consistently separated survivors from fatalities. The accumulation of lactate served as a sensitive and specific quantitative indicator of the severity of the oxygen dept and anaerobic metabolism. No patient survived in whom the arterial blood lactate was greater than 4 mmol/L for more than 12 h, regardless of the magnitude of the stroke volume, the left ventricular filling pressure, or the cardiac work. We regard the measurement of arterial blood lactate as a consistently useful prognostic indicator of survival or fatality in patients with acute myocardial infarction and myocardial failure.