An unusual pleomorphic tumor was resected from the cerebellopontine angle of a 40-year-old man with no stigmata of neurofibromatosis. The tumor showed multinucleated giant cells scattered amid smaller, slightly elongated cells. The tumor showed a rudimentary fascicular pattern containing some looser areas but no nuclear palisading. A diagnosis of Schwann cell tumor was made based on ultrastructural findings and on immunoreactivity for S100 protein. Malignancy was suggested by the extreme pleomorphism of the tumor, by the presence of tripolar mitotic figures, and by flow cytometric demonstration of aneuploidy and of a significant proportion of S-phase tetraploid nuclei (9%). Ten months after the first operation, the tumor had regrown to its original size, and was again resected. The histologic and ultrastructural appearances were similar to those of the first specimen, but flow cytometric analysis now showed a double aneuploid population of nuclei and showed 12% of the nuclei in tetraploid S phase. Within 11 months of the second operation, the tumor had regrown to an estimated size of 22 cm3. Four months of radiotherapy subsequently halted tumor growth, but additional lesions appeared in the cervical and high thoracic areas.