Ninety-two children underwent aortic (AVR) or mitral valve replacement (MVR), or both, at the Hospital for Sick Children in Toronto from 1963 to February, 1980. No early or late deaths occurred in 39 children having AVR. However, in 50 children having MVR, the operative mortality was 32% and the actuarial survival rate 5 years after operation was only 50%. Major complications occurred with almost equal frequency in the two groups; 50% of children surviving AVR or MVR experienced major complications within 6 years of operation. Retrospective comparison of results with tissue and mechanical valves showed no clear advantage with either type of prosthesis. Outgrowth of a prosthetic valve was satisfactorily managed in children with AVR but presented a difficult problem in those with MVR, who required frequent reoperation to increase the prosthetic valve to adult size. Prosthetic valve replacement in children is a palliative procedure at best, and every effort should be made to preserve the natural valve by a more conservative repair technique.