Approximately 70 cardiac fibromas (fibroelastic hamartomas) have been reported in the literature and at least 15 have been successfully excised. However there is no well-documented ultrastructural study of these lesions. A successfully excised cardiac fibroma (FEH) was studied by light and elctron microscopy. The tumor was composed mainly of fibroblasts admixed with bundles of collagen and elastic fibers. No muscle fibers were demonstrated in the central region of the tumor either by light or electron microscopy. Mast cells were occasionally present. The lack of encapsulation enabled fibroblasts with collagen and elastic fibers to extend between groups of myocardial fibers at the periphery of the tumor. Within these myocardial fibers ultrastructural changes were limited to the mitochondria and myofibrillar structure.