The commonest pineal gland tumor, the germinoma, is more apparently related to germ cells than to pineal cells. Although ultrastructural studies of this tumor have confirmed this similarity, comparable characterization of a pineocytoma has not been reported, due to its rarity. Here we define the electron microscopic features of a rapidly growing human pineocytoma and compare the tumor cells in the present study with normal mammalian pinealocytes, as well as with cells described in the literature of pineal body pathology. Two cell types populate the tumor: one resembles the neuron, while the second, less frequently encountered, is often indistinguishable from the fibrous astrocyte. Synaptic complexes and dense-cored vesicles were not conspicuous fine-structural elements. Whereas emphatic morphological differences exist between the germinoma and pineocytoma, the similarities in ultrastructure between the pineocytoma cells and adult mammalian pinealocytes are even more striking.