Mitral valve surgery was performed in 88 patients with severe pulmonary hypertension (average systolic pulmonary artery pressure, 94.7 +/- 22 mm Hg; range, 70-180 mm Hg) over a 10-year period. Sixty-four patients (73%) were in New York Heart Association Functional Class III or IV. There were 64 valve replacements and 24 open mitral commissurotomies. Operative mortality was 5.6% (5 patients) and was not related to the degree of pulmonary hypertension, surgical procedure performed, or type of valve lesion. A 100% follow-up was obtained, ranging from nine months to 10 years, with a mean of 44 months. Six late cardiac deaths (7.2%) occurred, 5 in patients with valve replacement and 1 in a patient who underwent a commissurotomy. Actuarial survival was 86 +/- 3% at five years and 83 +/- 4% at 10 years. Fourteen patients underwent right ventricular catheterization a mean of 24 months following operation. Systolic pulmonary artery pressure had decreased from a mean preoperative value of 101 +/- 22 to 40.5 +/- 7 mm Hg (p < 0.001). Cardiac index increased by 55% of the preoperative values. Functional status improved markedly; 71 survivors (93%) were in New York Heart Association Class I or II. These results indicate that, in patients with mitral valve lesions and severe pulmonary hypertension, (1) surgical procedures can be performed with an acceptable operative mortality; (2) excellent long-term survival and functional results can be obtained; and (3) pulmonary hypertension decreases significantly after operation. Patients with mitral valve disease may benefit from surgical treatment regardless of the degree of pulmonary hypertension.