Twenty-six cases of neurogenic tumors consisting of 2 ganglioneuromas (GN), 8 ganglioneuroblastomas (GNB), and 16 neuroblastomas (NB) were studied to evaluate their cytodifferentiation. Ultrastructurally, a moderate to large number of neuritic processes and high density neurosecretory granules (NSG) were found in all cases of GN and well-differentiated GNB, in two-thirds of poorly differentiated GNB, in about a half of rosette-fibrillary NB, and in no case of round-cell NB. All GN and GNB had tumor cells which were positive for both chromogranin and neurofilaments. Of the 16 cases of NB, 12 were positive for chromogranin, and 13 and 15 were positive for Mr 200,000 and 68,000 neurofilament polypeptides, respectively. However, both markers appeared mainly in tumor cells maturing toward neuroblasts. Electron microscopy was helpful for the diagnosis of undifferentiated NB in those cases immunohistochemically negative for chromogranin or neurofilaments. We conclude that ultrastructural and immunohistochemical examinations are useful for the morphologic assessment of the degree of maturation of neuroblastic tumors.