The ultrastructure of oral neuromas from 2 patients with multiple mucosal neuromas, pheochromocytoma, medullary thyroid carcinoma syndrome reveals numerous hypertrophic unmyelinated and myelinated axons, hyperplastic neurilemmal cells and associated collagen fiber formation. These tumors are described and compared ultrastructurally with neurilemomas and neurofibromas as described by other authors. On the basis of this comparison, these tumors are not considered to be of neurilemmal origin. On the basis of light and electron microscopic histology it is postulated that these tumors represent hypertrophy of axons similar to that noted in amputation neuromas.