Our previous studies indicate that glucose transporter 5 (GLUT5) is a microglial marker in routine paraffin sections, and is rarely present in monocytes/macrophages of the peripheral organs. We examined the expression of GLUT5 in 91 cases of human gliomas to characterize the microglial phenotype in glioma tissues. Immunohistochemistry was performed on formalin-fixed, paraffin-embedded sections using such antibodies as a GLUT5 antibody, two markers for activated microglia: major histocompatibility complex (MHC) class II Ag and macrophage scavenger receptor class A (MSR-A), and MIB-1 antibody. The immunoreactivity of GLUT5 was present in three microglial phenotypes: ramified (resting), activated, and ameboid (macrophagic) microglia in most of the cases. A double-labelling study of astrocytic tumours using GLUT5 and MIB-1 antibodies demonstrated a proportion of proliferating microglia. However, no morphological difference between MIB-1-positive, microglial cells and MIB-1-negative, microglial cells was found. The number of GLUT5-positive microglia was significantly (P < 0.001) higher in astrocytic tumours than in oligodendroglial tumours. Many GLUT5-positive microglia (up to 52% in total cells) were often observed in pilocytic astrocytomas, where microglial cells were predominantly ramified, and the number of MHC class II- or MSR-A-positive microglia was less than GLUT5-positive microglia. Thus, the present study indicated that intrinsic microglia can be a source of microglia/macrophages cell populations in astrocytic tumours, and that pilocytic astrocytomas often have a high proportion of microglial cells with mild activation.