To provide baseline information for the "local" therapy of the glioblastoma multiforme (GBM), whole-brain histological sections of 15 untreated GBM's were studied to determine the distribution of neoplastic cells. These findings were then compared with the computerized tomography (CT) scans in 11 cases in order to determine the extent to which the peripheral portion of the neoplasm can be estimated by the presence of a low-density area without contrast enhancement. The results of the histological study confirmed the marked heterogeneity of GBM's and disclosed a great variability in the geometry, extent, and character of the peripheral "infiltrating" margin. In spite of the widely held concept that glioblastomas are localized within 2 cm of the contrast-enhanced rim, there were three cases in this two-dimensional study in which this distance was exceeded, and it seems likely that three-dimensional reconstructions would have detected additional cases in which neoplastic cells extended beyond this arbitrary limit. Only three of the 15 GBM's were restricted to the distribution of one internal carotid or one vertebral artery. To the extent that the neoplasms in the present series are representative, this suggests that glioblastomas will be difficult to treat successfully by intra-arterial therapy using existing therapeutic agents. Correlations of histological sections with the CT scans revealed that the vast majority of the neoplastic tissue was contained within the contrast-enhancing and "peritumoral" areas of low density, but that in five cases fingers of neoplasm extended for short distances beyond the outer margin of the latter region. This indicates that the distribution of cells of a GBM cannot be inferred from CT images since the "peritumoral" area of low density can over- or underestimate the extent of the lesion.