Forty seven bioprostheses were used for mitral valve replacement between January 1975 and June 1980, with no operative mortality, in children under 19 years of age. This study was undertaken to evaluate the medium and longterm outcome of 43 patients followed up for at least 2 years. The late mortality was higher in children under 13 years of age (11.1% per patient/year) than in older children (3% per patient/year). The incidence of reoperation for deterioration of the bioprosthesis was 5,5% per patient/year. The actuarial longevity of bioprostheses without any complications was 48 +/- 16% at 5 years. After a review of the literature, the authors discuss their present therapeutic attitude: whenever possible, mitral valvuloplasty is the operation of choice, but when valve replacement is necessary, two criteria must be considered: the age of the patient and conditions of follow up. If medical follow-up facilities are good: mechanical prostheses are preferred in patients under 13 years of age: after puberty especially in girls, the bioprosthesis is the valve of choice. If medical follow-up facilities are poor: the valve of choice is a bioprosthesis at all ages because of the risk of thromboembolism and the relatively slow clinical aggravation in cases of bioprosthetic deterioration.