Sixty-eight patients have had mitral valve replacement with Ionescu-Shiley bovine pericardial bioprosthesis from 1981 to 1984. Thirty five patients were required reoperation due to primary tissue failure of the bioprosthesis, 10 (28.6%) were on emergency basis and 25 (71.4%) were on elective. Hospital deaths were two (20.0%) on emergency and four (16.0%) on elective. Clinical features of emergency cases were as follows: progressive congestive heart failure in eight, severe hemolysis with massive hemoglobinuria in eight and acute renal failure in four. Cusp tears were observed in all valves, 8.1 +/- 1.5 mm in emergency group and 4.2 +/- 3.6 mm in elective group. Neointimal ingrowth over Dacron cloth of the inner surface of the stent was absent in 29 valves (89.2%). Calcification was observed in seven valves, however these were not dominant causes of primary tissue failure. Clinical symptoms were correlated with structural deterioration of explanted valves. Structural deterioration of the bioprosthesis may occur suddenly and progressively. Our experience demonstrated that life-threatening prosthetic valve failure may occur with a relatively high incidence and that careful follow-up is needed for Ionescu-Shiley bovine pericardial bioprosthesis.