Despite the widespread use of glutaraldehyde-preserved porcine xenografts, severe short-term calcification of these valves has been infrequently reported. This report describes four cases of glutaraldehyde-preserved porcine xenografts in the mitral valve position in which severe calcification occurred within 17 to 25 months of implantation. All four patients were children, aged 13 to 15 years. The clinical presentation in all four cases occurred at a late stage when there was severe xenograft obstruction, with acute symptoms of cardiac decompensation in the presence of pulmonary hypertension and right heart failure. There was rapid cardiac deterioration resulting in a low output state and episodic pulmonary edema necessitating urgent mitral valve replacement. In only one case was there clear auscultatory evidence of severe mitral stenosis. Calcification of these xenografts occurred in the presence of normal serum calcium levels and was not related to infective endocarditis. Histologic examination of the calcified xenografts strongly suggested dystrophic calcification resulting from primary collagen degeneration. The exact cause is unclear, but it appears that glutaraldehyde-preserved porcine xenografts may produce severe short-term calcification with acute hemodynamic deterioration necessitating urgent valve replacement and that this accelerated calcification may be a complication in young persons,