The relationships between right atrial mean, pulmonary artery diastolic, mean, and wedge pressures, and stroke index were studied in 100 patients. The various pressures correlated fairly well with one another, but poorly with stroke index. However, this correlation improved when the values of individual patients were analyzed. Infusion of calcium chloride modified the correlation of the various pressures with the stroke index over a short period of time. During anesthesia, when pulmonary vascular resistances, systemic resistances and myocardial contractility were stable, both pulmonary artery mean and wedge pressure correlated significantly with the stroke index.