The clinical and pathological features of six patients with so-called "intraventricular oligodendroglioma" are reported. The tumor had no predilection for sex, and the patients' age at diagnosis ranged from 15 to 39 years. The lesions were located in the lateral and/or third ventricles. Total removal of the tumor was performed in three patients, and the remaining three underwent partial resection. Postoperative irradiation was given to five patients. A follow-up study revealed that five patients were free of recurrent tumor at 15 to 227 months after treatment, and one was alive with disease 25 months after surgery. Histologically, all tumors were composed of small uniform cells, with perinuclear halos and regular round nuclei. Tumor cells were sometimes arranged around nucleus-free fibrillary zones. Mitoses were infrequent. Ultrastructurally, neoplastic cells had round nuclei with dispersed heterochromatin and organelle-sparse cytoplasm containing occasional microtubules, 20 to 25 nm in diameter, and scattered dense-core vesicles, 100 to 200 nm in diameter. Cell processes containing dense-core and clear vesicles were frequently present. Thus, these neoplasms should be considered neuronal in origin, and should be classified as "intraventricular neurocytomas."