Pathological findings in 20 cases of glioblastoma multiforme were correlated with clinical histories and computerized tomographic (CT) scans. This was done to define the neoplasm in three stages: before treatment, during remission, and during recurrence. The untreated lesions were markedly cellular neoplasms composed predominantly of small anaplastic cells. The radiographic central region of low density was necrosis, the enhancing rim was a cellular zone of viable neoplasm, and the perilesional low-density area was edema with infiltrating tumor. In these 20 cases, all of the identifiable neoplasms lay within the zone of peritumoral edema or contrast enhancement, although small anaplastic cells may have been present in more distant regions. The lesions in remission were remarkable for their minimal mass effect, discrete nature, extensive necrosis, and content of large bizarre glia. The large cells were confined to the original tumor bed and were consistent with neoplastic cells inactivated and immobilized by radio- and chemotherapy. These lesions were accurately localized by CT scanning. The recurrent lesions were heterogeneous, but most were formed of widely disseminated small anaplastic cells. The highly cellular regions of such lesions could be localized by CT scanning, but CT could not detect less cellular foci in the cerebrum, cerebellum, or brain stem. In one patient, the contrast-enhancing lesions of "recurrence," were foci of radionecrosis, underscoring the difficulty in distinguishing this entity from recurrent neoplasm.