Replacement of the mitral valve was performed in 130 children less than 15 years of age—115 for rheumatic and 15 for congenital disease. Early mortality was 5.4%. Of 123 survivors, 112 were followed postoperatively for a mean period of nine years, with 80 of them on anticoagulants. Thromboembolism is the major late problem, with 25 thromboembolic accidents noted in 25 patients (22.3%). Anticoagulation was ineffective or lacking in 72% of them. Bacterial endocarditis was noted in five cases (4.4%) and a prosthetic leak in three (2.6%). Particular problems are observed in this group of young patients. First, recurrence of rheumatic fever in nine patients with an increasing severity of aortic valvar incompetence; second, with growth the prosthesis can become small as observed in six of our patients. A reoperation was needed in 15 patients, with one requiring two reoperations. Late mortality was 21%. Optimal timing for replacement of the valve is crucial. Appropriate anticoagulation and correct prophylaxis of rheumatic fever and bacterial endocarditis is mandatory. Conservative surgery is certainly preferable to valvar replacement, but if replacement proves necessary, large prostheses should be used.