Twenty-two consecutive critically ill surgical patients who had a Swan-Ganz thermodilution catheter inserted as part of their intensive care unit therapy were studied prospectively. Patients were divided into two groups. Group I consisted of 17 patients whose pulmonary artery diastolic pressure minus pulmonary artery wedge pressure (PADP-PAWP) gradient was less than 5 mm Hg or if greater than 5 mm Hg persisted for less than 12 hours. Group II was composed of 5 patients whose PADP-PAWP gradient was greater than 5 mm Hg for at least 12 hours or more. The two groups were equally matched in terms of risk factors that predispose to an elevated gradient or factors that falsely lower the gradient. Nine other physiologic parameters were measured in patients in Group II during the period of the elevated gradient and were compared with the same parameters measured in patients in Group I. An elevated pulmonary vascular resistance was the only parameter associated with an elevated gradient. An elevated PADP-PAWP gradient greater than 5 mm Hg that persisted for 12 hours or more was associated with a 60 per cent mortality rate.