Objective — to investigate the morphological and phenotypic features of cardiac myxoma (CM), to discuss argumentative issues on the sources of its growth. Subjects and methods. The investigation included 176 sporadic and familial myxomas originating from different cardiac chambers (96% from the atria). The histological, ultrastructural, and immunohistochemical features of the cells and vessels of this tumor were studied. Results. The morphological and phenotypic features of the embryonic endothelium resembling the cells of the primary cardiovascular network were found in the cells and vessels of CM. No signs of other mesenchymal derivatives were detected. Discussion. The authors anticipate that there is focal cessation of maturation of the embryonic endocardial endothelium and its persistence — hamartia. Postnatal proliferation of embryonic hamartia forms the tumor hamartoma. Differentiated cells may retain the phenotypic (but not morphological) features of closely related cells that have a common precursor. Conclusion. The results of the study have allowed the authors to consider CM to be hamartoma, a monohistioid benign embryonic endocardial endothelioma. Our findings do not exclude the possibility of developing CM from pluripotent mesenchymal cells.