Prominent features of moyamoya disease are fibrocellular thickening of the intima and enhanced angiogenesis. The pathogenesis of moyamoya disease is, however, unknown. Basic fibroblast growth factor (FGF) is an angiogenic factor as well as a potent mitogen for a number of cell types including vascular endothelial and smooth-muscle cells. In order to test the possibility that basic FGF takes part in the pathogenesis of moyamoya disease, the authors tested for the presence of this factor using a mouse monoclonal antibody against human recombinant basic FGF. The surgical specimens studied included two sections of the superficial temporal artery (STA) and four samples of dura mater from four patients with moyamoya disease. Surgical specimens were obtained from three patients with other diseases as control tissue. Sections of the STA obtained from the patients with moyamoya disease showed strong basic FGF immunoreactivity in endothelial and smooth-muscle cells, while control sections had only faint and scattered immunoreactivity. All sections of the dura mater obtained from the patients with moyamoya disease also revealed more intense immunohistochemical staining of basic FGF in meningeal and vascular cells than did control sections. These observations indicate that the amount of basic FGF is increased in the tissues of patients with moyamoya disease; thus, basic FGF may play an important role in the pathogenesis of moyamoya disease.