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Background and Purpose: 11C‐methionine (MET)‐PET is a useful tool in neuro‐oncology. The T2‐fluid‐attenuated inversion recovery (FLAIR) mismatch sign on MRI is a characteristic finding in lower grade gliomas with isocitrate dehydrogenase (IDH) mutations and the absence of the 1p/19q codeletion; however, the T2‐FLAIR mismatch sign has low sensitivity in differentiating gliomas and does not aid in identifying glioblastomas with IDH mutations. We therefore investigated the efficacy of the combination of the T2‐FLAIR mismatch sign and MET‐PET for accurately determining the molecular subtype of gliomas of all grades. Methods: The present study comprised 208 adult patients diagnosed with supratentorial glioma confirmed by molecular genetics and histopathology. The ratio of the maximum lesion MET accumulation to the mean normal frontal cortex MET accumulation (T/N) was measured. The presence or absence of the T2‐FLAIR mismatch sign was determined. The presence or absence of the T2‐FLAIR mismatch sign and the MET T/N ratio were compared between glioma subtypes to evaluate individual and combined utility in identifying gliomas with IDH mutations and no 1p/19q codeletion (IDHmut‐Noncodel) or gliomas with IDH mutations (IDHmut). Results: The addition of MET‐PET to MRI for the presence of the T2‐FLAIR mismatch sign improved diagnostic accuracy, with the area under the curve values increasing from.852 to.871 for IDHmut‐Noncodel and from.688 to.808 for IDHmut. Conclusions: The combination of the T2‐FLAIR mismatch sign and MET‐PET may provide improved diagnostic utility in differentiating gliomas according to molecular subtype, particularly in determining IDH mutation status. [ABSTRACT FROM AUTHOR]