From 1981 to 1987 just over 608 Ionescu-Shiley low-profile bovine pericardial bioprostheses were implanted at the Toronto Hospital. Twenty-four prostheses (11 aortic and 13 mitral) were surgically explanted from 1988 to 1990 from 20 adults (10 men and 10 women). Prosthesis failure was caused by primary tissue failure in 17 valves or by other mechanisms in seven valves. Variable degrees of tissue failure were also seen in four of the seven valves from the latter group. Primary tissue failure was characterized by fluid insudation between collagen bundles, para stent post tears (alignment stitch related, 20 valves), cusp perforation with prolapse, and calcification. The earliest cusp tears occurred at 28 months. Calcification (10 of 24 cases) was minimal in seven of 10 valves (occurring primarily at the margins of the torn cusp), moderate in two, and severe in one. Tissue overgrowth (pannus) was seen in all but three prostheses. Like its predecessor, the Ionescu-Shiley standard pericardial valve, this prosthesis failed at 2 to 5 years largely due to design-related (alignment stitch) causes and tissue degeneration. Calcification was less prominent, while tissue overgrowth (pannus) was more marked.