A cardiac myxoma of the left atrium was studied by means of the electron microscope. A single cell type was seen, occurring predominantly as closely-apposed groups of 3 to 6 cells, and characterized by extensively anastomosing microvilli forming peripheral intracytoplasmic membrane-bound spaces (some of which may have been the result of cellular secretory activity), prominent linear and parallel roughsurfaced endoplasmic reticulum, variable electron-dense intracytoplasmic deposits, and poorly developed and sparse mitochondria and Golgi apparatuses. No fibroblasts, smooth muscle cells, or vascular channels lined by endothelial cells were present, although these have been reported in some of the earlier electron microscopic studies of cardiac myxomas. An endocardial subendothelial reserve cell origin of this tumor appears most likely.