We report the results and long-term follow up in 273 children (aged 2 to 16 years) who underwent prosthetic valve replacement. Mechanical valves (mostly Starr-Edwards) were used in almost all, and in 62 children more than one valve was replaced. Operative mortality was 4.7%. Actuarial survival curves (including hospital mortality) indicate a 86% survival rate at 5 years and 75% at 10 years. For isolated mitral valve prostheses (the largest subgroup), the figures are 87% at 5 years and 82% at 10 and 15 years. The main complication was thromboembolism, which occurred at a linearized rate of 2.7 per 100 patient-years. Actuarial curves indicate that 88% of patients are embolus free at 5 years, and 77% at 10 years. No patient with aortic valve replacement only had an embolism. Five of eight tricuspid prostheses thrombosed. Patients given aspirin and dipyridamole only did not have a higher rate of thromboembolic events than those given warfarin. There were five cases of endocarditis (two fatal) and four of dehiscence. No patient so far has needed replacement of a prosthesis because of somatic growth. Thus valve replacement can be performed with low mortality in children, and with satisfactory long-term survival. Thromboembolism remains a significant problem, although it appears to be less common than in adults. In this study, treatment with antiplatelet drugs only did not carry a higher rate of thromboembolic events than did treatment with warfarin.