A prospective study of cor pulmonale in 74 patients relates pulmonary haemodynamics to survival. Mean arterial oxygen tension (PaO2) was 7.2 +/- 0.14 kPa and mean arterial carbon dioxide tension (PaCO2) 6.6 +/- 0.12 kPa at entry. Pulmonary artery pressure (PAP), pulmonary vascular resistance (PVR), and cardiac output (CO) sitting and supine and where possible on exercise, were measured every one or two years by a floating catheter technique. Survivors showed very little change in pulmonary artery pressure, pulmonary vascular resistance or cardiac output. Those who did not survive, showed a steady increase of PAP and PVR, whether or not they had received continuous oxygen therapy at home. Cardiac output remained normal or slightly elevated despite increasing pulmonary artery pressure. The relationship between VO2 (minute oxygen consumption) and cardiac output remained within the normal or greater than normal range, even on exercise. Although a deteriorating clinical situation may be paralleled by changes in pulmonary haemodynamics, it is questioned whether such changes are causally implicated in mortality.