We have encountered a series of 8 third ventricular neoplasms with a distinctive chordoid appearance that appear to represent a clinicopathologic entity. The tumors occurred in 7 females and 1 male, ranging in age from 31 to 70 years. In all cases, imaging studies showed a large well-circumscribed third ventricular mass; a cystic component was noted in 2. The tumors consisted of cords and clusters of cohesive, oval-to-polygonal epithelioid cells with abundant eosinophilic cytoplasm, relatively uniform round-to-oval nuclei, and inconspicuous nucleoli. Mitotic activity was absent. The stroma consisted of scant, coarse fibrillar processes, as well as prominent, slightly basophilic, extracellular mucin resembling that in chordomas. Throughout the tumor, and surrounding its well-defined borders, were infiltrates of mature lymphocytes and plasma cells. Russell bodies were prominent in the latter. Adjacent brain tissue showed reactive changes with gliosis and numerous Rosenthal fibers. Immunohistochemically, tumor cells were strongly reactive for GFAP and vimentin, but negative or only weakly staining for EMA. The MIB-1 labeling index was approximately 1%. Ultrastructural examination of 4 cases revealed focal microvilli, scattered "intermediate" junctions, and focal basal lamina formation. Neither desmosomes nor cilia were seen. Total resections were achieved in 2 cases; only subtotal removals were achieved in 6. Subsequent tumor enlargement was noted in 3 of the 6 patients with incomplete resection, and of these, two died at post-operative intervals of 8 months and 3 years. The other patient survives 4 years post-operatively with stable residual disease. Of the 2 patients with total resection, 1 was lost to follow-up; the other, during a brief follow-up period, did well without evidence of recurrence.