The authors present a series of 10 cases of OCM which were treated by replacement of the mitral valve. Mitral incompetence was present in all patients, and was major or massive in 7 of them. Direct surgery to the mitral valve area has demonstrated pathological lesions in the mitral valve mechanism in 7 patients. A formal indication for mitral valve replacement in the surgical treatment of cases of OCM therefore exists when there is major mitral incompetence, intraventricular conduction defect, cardiac failure, or failure of previous myotomy.