Background When mitral valve repair in children is not possible, mitral valve replacement (MVR) is an alternative, which could result in better outcomes. We report the long-term outcome after replacement of systemic atrioventricular valve with bileaflet mechanical valves in annular position in patients less than 5 years of age. Methods Between 1983 and 2008, 29 MVRs were performed in children aged 1.82 ± 1.34 years (range, 95 days to 4.6 years); the average weight was 8.06 ± 2.92 kg (range, 4.7 to 15 kg). The main indication (86%) was severe regurgitation. Prosthesis size ranged between 17 mm and 25 mm. Results The follow-up was completed and totaled 373 patient-years (12.4 ± 8.6). Four children presented with permanent atrioventricular block. Freedom from thromboembolic and hemorrhage events at 1, 5, 15 years was 100%, 96.1%, and 91.3%, respectively. All deaths occurred within 2 months after MVR and were 5 children (17%). No late mortality was observed. Freedom of redo MVR at 5, 10, and 15 years was 95%, 86%, and 70%, respectively. At MVR, weight 7 kg or less and age 1 year or less ( p = 0.03) were associated with redo MVR. During redo MVR, a larger mechanical valve was implanted (+2 sizes), with no postoperative death. Currently, 23 patients are in New York Heart Association class I, and 1 patient is in class II. Conclusions The results of mechanical annular MVR are acceptable in children. Anticoagulants are well tolerated, with little thromboembolic complication. Long-term results for patients who survive the hospital period are excellent, and reinterventions show safe results.