Between March, 1971, and September, 1975, glutaraldehyde-stabilized pericardial xenografts were used for single valve replacement in.212 patients (142 aortic, 67 mitral, and three tricuspid). The 195 operative survivors were observed for a total of 5,926 months over a period 6 to 61 months (mean 30). actuarial analysis of late results indicates an expected survival rate at 5 years of 92.3 per cent for patients with aortic and 91.1 per cent for patients with valve replacement. The rate of systemic embolism has been 0.62 episodes per 100 patient years for the aortic and 2.48 episodes per 100 patient years for the mitral group in the absence of anticoagulant treatment. All six emboli occurred early postoperatively, were trivial or mild, and left no sequelae. Symptomatically, 96.7 per cent of patients are now in Class I and 3.3 per cent in Class II (N.Y.H.A.). Maintenance of structural and functional integrity of the glutaraldehyde-stabilized pericardial zenograft was demonstrated by histologic and hemodynamic investigations. Catheterization showed substantial circulatory improvement in both patients with aortic and those with mitral replacement. The transaortic gradients were negligible (8 mm. Hg at rest and 17.5 mm. Hg during exercise). The available indicates that results of valve replacement withpericardial xenografts. Over this period of follow-up, compare very favorably with those obtained with other available prostheses and tissue valves.