Abstract Observations on the ultrastructural morphology of benign mediastinal neurolemmoma, neurofibroma, and ganglioneuroma are presented. Distinguishing features of each tumor are described as observed in this study and reported in the literature. In neurolemmoma, the Antoni type A cells exhibit a cellular differentiation totally different from that of the Antoni B cells. The Antoni type A cells are characterized by numerous thin cytoplasmic processes emanating from a cell body with a narrow rim of cytoplasm. The Antoni type B cells lack these processes and have abundant cytoplasm which possesses a complex system of organelles. The extracellular compartment of Antoni type A tissue is composed of Iacunae outlined by lamina similar in appearance to the basement membrane surrounding the principal tumor cells. Neurofibroma is distinguished by elongated cells with a few thick cytoplasmic processes interspersed with occasional myelinated and unmyelinated axons in an extensive collagenous stroma. The principal tumor cells are distinguished from fibroblasts by the basement membrane coating their plasmalemma. Ganglioneuroma is characterized by the presence of large ganglion cells and their satellites and a vast number of myelinated and unmyelinated axons. A well-defined basement membrane coats the tumor cells. The small, membrane-bound osmiophilic granules encountered in the cytoplasm of ganglion cells have been observed in other tumors arising in sympathetic ganglia and paraganglionic cells.