In the 10-year period 1972-1981, 35 cardiac myxomas from 34 patients were studied. 29 tumors were surgical specimens and 6 were necropsy observations. No patient was under 10 years and only 3 were in the second decade. 25 growths were located in the left atrium, 6 in the right atrium and 2 in the right ventricle; 1 patient presented with myxomas involving both atria. The surface examined in 31 tumors appeared villous in 15, smooth in 13 and slightly irregular in 3. The tumor cells were enmeshed in Alcian blue positive mucoid material and presented as isolated, aggregated in clusters or in small cords, or lining vascular-like spaces. Ultrastructural examination performed in 9 cases disclosed a few features of endothelial cells: microvilli, cell interdigitations, desmosomes and pinocytotic vesicles; cytoplasmic organelles were usually more numerous in the cells arranged in clusters and lining spaces than in those isolated or in small cords; well-developed Golgi complex and endoplasmic reticulum were frequently observed and suggest a secretory function for the myxoma cells. The findings herein reported together with those of recent cytochemical studies support a mesenchymal origin for the myxoma cells with differentiation to endothelial line.