The reduction of pulmonary hypertension that occurs within 24 hours of valve replacement for mitral stenosis is well documented, but patients who die after surgery have not been adequately studied. Clinical and autopsy data for 16 patients who died following mitral valve replacement were reviewed. The emphasis was on preoperative and postoperative pulmonary arterial pressure and pulmonary vascular disease, including arterial, venous, and capillary changes. Morphologic features were graded and summed to obtain an additive histologic assessment (AHA). Patients were divided into three groups: 1) those who had uneventful operations and early postoperative periods but died prior to discharge; 2) those who had postoperative difficulty, with identifiable acute anatomic causes of death; and 3) those who had postoperative difficulty, with no apparent acute anatomic cause of death. In group 1 (n = 4) the preoperative pulmonary arterial pressure was 43 +/- 17 mm Hg, and AHA ranged from 0 to 4; in group 2 (n = 5) the preoperative pulmonary arterial pressure was 60 +/- 15 mm Hg, but AHA ranged only from 2 to 5. In group 3 (n = 7) the preoperative pulmonary arterial pressure was 59 +/- 12 mm Hg; AHA ranged from 6 to 9, significantly higher than that of the other groups (P less than 0.005). Three patients from group 3 had elevated pulmonary arterial pressure (60, 52, and 50 mm Hg three, six, and 15 days after surgery, respectively). Two additional patients had right heart failure with normally contracting left ventricles terminally. It is concluded that some patients with mitral stenosis who die after surgery with persistently elevated pulmonary arterial pressure have sufficiently severe pulmonary vascular disease to account for their persistent pulmonary hypertension and death.