THE CAUSE OF moyamoya disease remains unknown, and pathophysiological mechanisms remain uncertain. Basic fibroblast growth factor (FGF) is a pluripotent polypeptide that has been shown to play roles in angiogenesis, tumorigenesis and many other processes. In a previous study, we demonstrated immunohistochemically that the amount of basic FGF was increased above normal in the superficial temporal artery (STA) of patients with moyamoya disease. To clarify the function of basic FGF in moyamoya disease, we have performed an immunohistochemical study of the STA using a polyclonal antihuman FGF receptor antibody and also have tested immunohistochemical reactions for basic FGF. Twelve surgical specimens of the STA from patients with moyamoya disease were studied. Twelve specimens of the STA from skin flaps of patients with other neurological diseases were also investigated for comparison. The sections of the STA from patients with moyamoya disease showed dense and strong FGF receptor and basic FGF immunoreactivity in endothelial cells, in cells scattered in the thickened intima, and in smooth muscle cells in the media. In contrast, the sections of the STA of control patients showed faint basic FGF immunoreactivity. The statistical analysis revealed a significant difference of basic FGF immunoreactivity between moyamoya disease and other neurological diseases (χ2= 23;P= 0.0001). Moderately intense FGF receptor immunoreactivity was observed in most control patients. However, the statistical analysis revealed a significant difference of FGF receptor immunoreactivity between moyamoya disease and other neurological diseases (χ2= 13.382;P= 0.0012). Results of these experiments indicate that the amounts of basic FGF and FGF receptor are increased in the STA of patients with moyamoya disease and that both basic FGF and its receptor exist in smooth muscle cells. Thus, smooth muscle cells with abundant basic FGF may stimulate themselves via an autocrine mechanism and migrate to and thicken the intima of patients with moyamoya disease.