Hemodynamic monitoring was performed in 100 acutely ill patients admit ted to a Coronary Care Unit, 72 of whom had sustained an acute myocardial infarction (AMI). In patients with AMI, the initial pulmonary capillary wedge (PCW) pressure was related to prognosis, with a mortality of 68% in those with elevated pressures. The mean PCW pressure was 25 mmHg in those who died as compared with 17mmHg (p < 0.001) in the survivors. Cardiac catheterization confirmed the clinical impression of high risk in the majority of cases but also identified those in whom the clinical signs were misleading; 9 of those with AMI (12.5%) had high PCW pressures in the absence of clinical or radiological heart failure. The diagnosis of serious hemodynamic complications of AMI (right ven tricular infarction, ventricular septal defect, and mitral regurgitation) was es tablished in 14 cases (19%). There were no serious complications related to catheterization, and we conclude that pulmonary artery catheterization is safe and of clinical value in acutely ill cardiac patients.