In order to establish the influence of prosthetic valve replacement on the natural course of mitral, aortic, and double-valve disease of NYHA class III and IV, the progress of 359 patients with Björk-Shiley mitral-valve prostheses (BSM), 317 with aortic valve prostheses, and 107 with double valve prostheses was compared with that of patients who had been treated medically. In this last group of patients, valve replacement had been recommended in the period 1968-1976, but for various reasons had not been carried out. Cumulative survival rates after 8 years were 77% for the BSM group, but only 32% for the medically treated group (p less than 0.0001). One year after operation, 70% of the BSM patients showed an improvement equivalent to one NYHA class, and in the majority this was still apparent after 5 years. In patients with aortic-valve incompetence, cumulative survival rates were 86% for the surgically treated group and 32% for the medically treated group (p less than 0.00001). In aortic-valve stenosis, these differences of cumulative survival rates were even more pronounced (p less than 0.000001), and were calculated to be 85% in surgically treated and 10% in medically treated patients. Clinical improvement in the BSA group averaged 1.5 NYHA classes. The 5-year survival rate for the patients with mitral and aortic-valve disease was 32%, while following doublevalve replacement it was 67% (p less than 0.005). Clinical improvement after double-valve replacement was similar to that following mitral-valve replacement. Prosthetic heart-valve replacement significantly prolongs life in patients with isolated mitral- or aortic-valve lesions as well as in patients with double-valve disease of NYHA classes III and IV. Even in the early years of the study, when the operative mortality was relatively high, the surgically treated groups had a significantly higher survival rate than the conservatively treated groups, and this was already apparent at an early stage in the follow-up period. Improvement of functional capacity and relief of symptoms amount to a difference of approximately one NYHA class in mitral and double-valve disease and of approximately 1.5 NYHA classes in aortic-valve disease.