A boy with familial eosinophilia had the hypereosinophilic syndrome, with involvement of mitral and tricuspid valves. Between the ages of 11 and 20 years, he underwent eight surgical procedures on his atrioventricular valves. The pathology included recurrent thrombotic vegetative masses related to hypereosinophilia. Initial repair of the mitral valve was shortlived, but recurrent repairs of the tricuspid valve were helpful. Mechanical prostheses inserted in the mitral position thrombosed despite anticoagulant therapy, and bioprosthetic valves deteriorated with thrombus, fibrosis, or tearing. The hypereosinophilic syndrome is unusual in children, and produces additional problems with valvar surgery.