Although there have been diverse opinions, permanent anticoagulation is generally recommended in children with mechanical valves. We experienced the thrombotic dysfunction of mitral St. Jude Medical valve in a 4-year-old child who had been under control of anticoagulation with warfarin sodium. Besides the primary problems related to the prosthetic mechanism, various patient factors might have contributed to the valve thrombosis. We would emphasize the necessity of anticoagulation and the close observation for valve dysfunction in pediatric patients with St. Jude Medical valve in mitral position.