From June, 1974, through June, 1978, 547 porcine xenografts (299 mitral, 239 aortic, eight tricuspid, and one pulmonary) were inserted in 459 selected patients who survived the operation and were considered at risk for primary tissue valve failure. Ninety-two instances of primary tissue valve degeneration occurred in 82 of the 459 patients. In patients operated on 10 years ago, 28% of the valves implanted in the mitral position (5/18) and 33% in the aortic position (4/12) failed. These percentages decreased to 24% (21/89) and 26% (15/57) for those implanted in 1975, 15% (13/85) and 16% (9/57) for those implanted in 1976, 13% (9/71) and 15% (12/82) for 1977, and 8% (3/36) and 3% (1/31) for 1978. None of the tricuspid or pulmonary valves have failed. The rate of valve survival without primary degeneration was 70% +/- 6% for the mitral and 69% +/- 7% for the aortic prostheses at 10 years of follow-up. Of the 10 valves implanted in patients younger than 20 years of age, 40% of the mitral and 60% of the aortic valves failed. This rate is 26% for mitral and 42% for aortic for the 21 to 30 year group, 20% and 19% for 31 to 40 years, 9% and 13% for 41 to 50 years, 20% and 6% for 51 to 60 years, and 11% and 0% for 61 to 70 years. The average interval between placement and explantation or death was 74 months (range 10 to 112) for valves in the mitral position and 78 months (range 13 to 117) for valves in the aortic position. We believe these current data should have some bearing of practical import not only when comparing different types of bioprostheses but especially when recommending the appropriate valve replacement for the individual patient.