In a series of 568 patients with glutaraldehyde-preserved porcine xenografts followed for up to 3 1/2 years, calcification and stenosis of the xenograft has proved to be an important cause of valve failure. We have seen this complication only in children and adolescents. Seven of 54 patients under 16 years of age at the time of initial operation have required reoperation for calcification and prosthetic stenosis. Three patients who died had calcified, stenosed prosthesis at autopsy. The calcification has involved all of the cusps of the affected prostheses, and the calcified area often was enclosed by a superficial layer of noncalcified cuspidal tissue. We no longer insert xenograft valves in young patients.