A 33-year-old woman had an undifferentiated tumor originating in the cerebral dysgenetic lesion resembling fibrous cartilage. She had a headache, vomiting, late-onset epilepsy and left hemiparesis. The lesion was located in the right temporal lobe on computed tomographic (CT) scan. It was totally resected and only local irradiation was performed postoperatively. Normal cortical architectures were lost in the resected specimens. Straight or curved fasciculi composed of fine collagen fibers were distributed in parallel and perpendicular to the cortical surface in the mildly eosinophilic hyaline matrix. Hypertrophic astrocytes were scattered with low cellularity in these abnormal cortices. Clusters of tumor cells were observed in a few areas. The tumor cells, having oval and relatively vesicular nuclei with a few prominent nucleoli and basophilic well-circumscribed narrow cytoplasm, had proliferated diffusely with a cobblestone appearance. Immunohistochemical and electron microscopic investigations demonstrated no evidence of specific differentiation, either. There were 14.5 mitotic figures/high power field on average and most nuclei of the tumor cells were strongly positive for proliferating cell nuclear antigen (PCNA). Moreover, subarachnoid dissemination of the tumor cells were apparent in a few areas. Nevertheless the patient returned to work and no recurrence was observed for 10 years postoperatively. We concluded that neoplastic transformation occurred de novo in the dysgenetic cortex.