The role of circulatory congestion in the cardiorespiratory dysfunction of massive obesity was investigated in 18 patients. They were hypervolemic and had increased cardiac outputs proportionate to their weight. The average resting left ventricular filling pressure was within the upper limits of normal, but it increased to abnormally high levels with increased venous return of passive leg raising, and further during exercise. The elevations in pressure were associated with high resting central blood volumes which increased significantly with exertion. These findings are consistent with reduced distensibility of the central circulation in these congested patients. Weight reduction was accompanied by a decrease in central blood volumes and restoration of a normal left ventricular response in three of four patients and a return toward normal in one. The improvement in ventricular function with relief of edema and dyspnea. In 14 patients with normal or only minimal alveolar hypoventilation, there were no significant transpulmonary diastolic pressure gradients despite a marked increase in left ventricular end-diastolic pressures. One patient, after regaining weight, subsequently had an abnormal gas exchange and an increased pulmonary vascular resistance. He and two others with severe alveolar hypoventilation demonstrated cor pulmonale on a background of left ventricular dysfunction and congestion of the circulation. Two other patients, the least obese of the group, had hypoventilation and cor pulmonale with normal left ventricular pressures. Hypervolemia and a hyperdynamic state are common features of the obese patients. High cardiac output is maintained despite marked circulatory congestion which may result in generalized anasarca and increased ventricular filling pressures. This clinical syndrome may be present in obese patients without intrinsic heart disease and may be reversible with weight reduction. The central circulatory congestion may contribute to the development of the alveolar hypoventilation syndrome in certain obese patients.