Hematopoietic, glandular, and mesenchymal elements can be found within cardiac myxomas; ectopic endocrine tissues and "thymic rests" have also rarely been described. Atrial tumors (one right and one left) from 2 patients (a 69-year-old man and a 77-year-old woman) were encountered among the atrial myxoma cases in one of the author's consultation files. Both tumors were comprised of classic cardiac myxoma (with characteristic rings and syncytial chains of myxoma cells in a loose myxoid matrix) and cellular thymoma-like elements (characterized by a lobulated sheet-like growth of epithelioid spindle cells admixed with small lymphocytes punctuated by vessels with prominent perivascular spaces). Neither patient had evidence of thymoma elsewhere. Immunophenotypically, the thymoma-like component reacted strongly with antibodies to keratins (AE1/AE3, Cam 5.2, wide spectrum, CK19, CK7) and CD57 and weakly with antibodies to CD31, CD34, and calretinin. This intermediate phenotypic expression of both epithelial and vascular antigens likely reflects the multipotential nature of the cells comprising this lesion. The most likely explanation for this extremely unusual finding is neoplastic transformation of thymic rests within a myxoma.