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Studies of experimental tumorigenesis have strongly implicated signaling of the insulin-like growth factor 1 (IGF-1) as a key component in astrocytic neoplasia; however, its role in the growth of low-grade and malignant human tumors is not well understood. Correlative analyses of IGF-1, p53, and Ki-67 (MIB-1) immunohistochemistry and IGF-1 receptor (IGF-1R) mRNA expression were performed to examine the cellular pattern of IGF-1 signaling in 39 cases of astrocytoma (World Health Organization grades II-IV). Tumor cells expressing IGF-1 and IGF-1R were present in all tumor grades. The proportion of tumor cells that expressed IGF-1 correlated with both histopathologic grade and Ki-67 labeling indices, while expression of IGF-1R mRNA correlated with Ki-67 indices. In cases where stereotactic tissue sampling could be identified with a specific tumor area by neuroimaging features, the numbers of IGF-1 immunoreactive cells correlated with the tumor zones of highest cellularity and Ki-67 labeling. In glioblastomas, the localization of IGF-1 immunoreactivity was notable for several features: frequent accentuation in the perivascular tumor cells surrounding microvascular hyperplasia; increased levels in reactive astrocytes at the margins of tumor infiltration; and selective expression in microvascular cells exhibiting endothelial/pericytic hyperplasia. IGF-1R expression was particularly prominent in tumor cells adjacent to both microvascular hyperplasia and palisading necrosis. These data suggest that IGF-1 signaling occurs early in astroglial tumorigenesis in the setting of cell proliferation. The distinctive correlative patterns of IGF-1 and IGF-1R expression in glioblastomas also suggest that IGF-1 signaling has an association with the development of malignant phenotypes related to aberrant angiogenesis and invasive tumor interactions with reactive brain.