A 41-year old man, with idiopathic hypereosinophilic syndrome, had severe mitral regurgitation. He had two subsequent mechanical mitral valve thromboses in the setting of rising eosinophilia. A Carpentier-Edwards bioprosthesis was used for the third valve replacement. Some features of myeloproliferative disease, including chromosome 10 abnormality, were identified. A normal eosinophil count and disappearance of the chromosomal abnormality were obtained with interferon, hydroxyurea and prednisone therapy. At 30 months postoperatively, the patient had a normal lifestyle.