Pulmonary extravascular water volume (PEV) or lung water was measured in 45 patients with acute myocardial infarction, utilizing the double radioisotope indicator dilution technique. A PEV greater than the upper limit or normal (120 ml/m2) was found in 27 patients, 23 of whom had an elevated pulmonary wedge or pulmonary artery diastolic pressure. A significant correlation was found between PEV and pulmonary wedge or pulmonary diastolic pressure in the whole series. A progressive increase in average PEV was observed from Class I (uncomplicated) to Class IV (shock) patients. There was also a progressive increase in PEV from patients with normal chest X ray findings to those with radiologic evidence of acute pulmonary edema. In seven of 11 patients with initial elevation of both PEV and pulmonary wedge pressure, repeat determinations demonstrated a substantial reduction in both parameters over a period of 2-4 days. It is postulated that an increased PEV in patients with acute myocardial infarction is largely due to an elevated pulmonary capillary pressure. The latter is probably a consequence of an elevated left ventricle diastolic pressure, which may be a manifestation of either left ventricular failure or a decrease in left ventricular compliance.