We report a case of myxoid leiomyosarcoma originating from the interventricular septum in the outflow tract of the right ventricle. Although the gross features suggested a benign myxoma, histologic examination demonstrated features of a smooth muscle tumor, which was characterized by low mitotic index and a bland degree of atypia with few cells immunoreactive for cell cycle-associated Ki-67 antigen. The tumor relapsed twice, and the patient (a 61-year-old woman) died 18 months after the first diagnosis. This case demonstrates that myxoid leiomyomatous proliferations of the heart must be considered potentially malignant, even when the gross features and degree of cellular atypia seem to suggest otherwise.