[Show abstract][Hide abstract] ABSTRACT: Glial fibrillary acidic protein (GFAP) is used widely as a specific marker of astrocytes, in part because GFAP-positive cells have rarely been encountered outside the central nervous system. In this paper we describe a 16-year-old female with a brain stem glioblastoma and GFAP immunoreactive cells in areas of bone marrow with osteosclerotic changes. These GFAP cells were negative for S-100 and devoid of cell atypism, showing non-neoplastic features. To investigate the factors which might be involved in the induction of GFAP in the non-neoplastic cells, we examined the expression of cytokines and growth factors in the bone marrow. Immunoreactivities of interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) were found to be reduced. In contrast, those of epidermal growth factor receptor (EGFR) were increased. Glial fibrillary acidic protein expression with decreased IL-1β and TNF-α and increased EGFR in bone marrow may be related to the pathogenesis of progressive osteosclerosis found in this patient with brain stem glioblastoma.