From August 1977 to June 1981, 221 patients received a Hancock porcine valve and 133 an Ionescu-Shiley bovine (I-S) pericardial valve as aortic valve substitutes. No special selection or randomization was used and no patient with either of these types of valves was excluded. Preoperative data show no differences between the groups influencing the appearance of primary tissue failure. Hospital survivors were followed until June 1984 and those with an uneventful history at least 36 months. Patients who died late postoperatively or who underwent reoperation for causes other than primary dysfunction were considered at risk until death or reoperation. Primary tissue failure occurred in 8 patients in the I-S group from 36 to 70 months postoperatively and in 6 patients of the Hancock group from 24 to 83 months. Linearized rates of primary failure were 0.61 valves per 100 patient-years for the Hancock and 1.70 valves per 100 patient-years for the I-S group. Mean age of patients with failing valves was 38 years (range 25 to 55) for Hancock valves and 39 years (range 15 to 62) for I-S valves. Actuarial analysis shows a lower rate of primary dysfunction in the Hancock group since the fourth year, which is statistically significant in the sixth and seventh years (96.5 +/- 1.5% vs 79.6 +/- 7.6% in the sixth year and 93.1 +/- 3.6 vs 79.6 +/- 7.6% in the seventh year). Microscopically, calcium and collagen degeneration were consistently associated and present on failing bioprostheses.(ABSTRACT TRUNCATED AT 250 WORDS)