Nine children ages 2-15 years have undergone mitral valve replacement with Hancock porcine heterograft valves at Stanford Hospital. Six patients had isolated mitral valve abnormalities (congenital or rheumatic) while 3 had complex congenital heart disease. All had severe mitral insufficiency and 5 had heart failure. Porcine mitral valve sizes utilized ranged from 19-31 mm. There were no surgical deaths, and all patients were markedly improved on discharge. Postoperative follow-up has been 1-5.5 years (mean 3.7 years). None of the children has had bacterial endocarditis, and none has had thromboembolic complications despite no long term anticoagulation. Seven of the 9 patients have remained asymptomatic. The other 2 have required valve re-replacment 3.6 and 4.8 years postoperatively because of severe fibro-calcific obstruction but are now asymptomatic 2 and 5 months later. In marked contrast, only 2 of 453 Stanford adult patients developed obstruction of heterograft mitral valves which required re-replacement. At Stanford, the Hancock valve remains the preferred valve for mitral replacement in children because it, a) can be performed with low risk, b) provides good functional improvement, and c) ordinarily requires no long term anticoagulation. Stenosis of the porcine heterograft does occur and deserves particular attention because it may be a more frequent complication in children than in adults.