Eight children (mean age, 7.6 years) had xenograft prosthetic valves inserted. All developed evidence of prosthetic valve obstruction requiring reoperation 18 to 55 months after insertion (mean, 32 months). At operation, all the valves were found to be heavily calcified. There were two perioperative deaths. Of two patients who had a second xenograft valve inserted, one died suddenly 12 months later. Gross calcification of the xenograft was found at postmortem examination. The other patient had evidence of mild xenograft calcification at follow-up 19 months postoperatively. Four of the ten calcified xenograft valves were of bovine origin (Ionescu-Shiley type); in these the calcific obstruction occurred significantly earlier than in the six porcine (Hancock) valves.