Isocitrate dehydrogenase 1 (IDH1) mutations are common in grade II-III diffuse gliomas and secondary glioblastomas. The aim of this study is to investigate the staining pattern of mIDH1R132H, an antibody specific to mutant IDH1 protein, in primary brain tumors and non-neoplastic systemic organs. Eight of 13 diffuse astrocytomas, 1 of 6 anaplastic astrocytomas, 9 of 11 oligodendrogliomas, 15 of 22 anaplastic oligodendrogliomas, 6 of 7 oligoastrocytomas, and 5 of 8 anaplastic oligoastrocytomas showed both cytoplasmic and nuclear positivity. Two of 25 atypical meningiomas and 2 of 42 pituitary adenomas were positive for mIDH1R132H. The following non-neoplastic systemic organs showed positivity in the cytoplasm alone: the myocardium, peribronchial glands, interlobular ducts of the salivary gland, gastric fundic gland, columnar epithelia of the large bowel, hepatocytes, centroacinar cells, the intercalated ducts of the pancreas, renal proximal and distal tubules, adrenocortex, ovarian granulosa cells, and the choroid plexus epithelia. It was concluded that the immunopositivity detected in non-neoplastic systemic organs was due to cross-reactivity, because immunohistochemistry with an anti-mitochondrial antibody revealed that the mIDH1R132H staining pattern was identical to that of the mitochondria. Therefore, mIDH1R132H positivity should only be considered to be validated when a cell shows both cytoplasmic and nuclear staining.