Five hundred and eighty six patients with mitral valve disease were studied with cardiac catheterization between 1961 and 1972; 48 (8.2%) had extreme pulmonary hypertension (resting systolic pulmonary artery pressure of 80 mmHg or above and pulmonary vascular resistance of 10 units or greater) and of these patients, 27 underwent cardiac surgery. The operative mortality for mitral valvotomy was 11 per cent and for mitral valve replacement 56 per cent. The overall mortality was 31 per cent. The risks of operation were increased in those with a long history of cardiac symptoms, those over 50 years of age, and in the presence of associated aortic valve disease. The mean survival for those patients not having operation was only 2.4 plus or minus 0.5 years. The mean follow-up period for those surviving operation has been 5.8 plus or minus 0.6 years, and symptomatic improvement has been good.