A patient with seizures and a contrast-enhancing temporal lesion after radiation therapy for a chondrosarcoma of the nasal septum is described. To differentiate between radiation necrosis and recurrent tumor, thallium-201 (201Tl) SPECT was used. 201Tl SPECT revealed high local accumulation suggesting tumor growth; however, pathologic examination demonstrated focal necrosis with reactive changes but without tumor. The 201Tl SPECT findings in this patient demonstrate a possible diagnostic pitfall in differentiating recurrent tumor from radiation necrosis.