The characteristics of clear cells of an ependymoma of the foramen of Monro have been studied by electron microscopy to precisely define its organellar composition and to establish the tumor histogenesis. Our data confirm that the once-thought oligodendroglial is, in fact, an ependymal tumor. Both the scarce number of organelles, owing to the low degree of differentiation, and the abundance of hyaloplasmic lipid vacuoles can account for the clear appearance of these tumor cells.