WHO criteria now classify adult glioma into groups using combination of tumor grade and two tumor markers: 1p/19q codeletion and IDH1/2 mutation. TERT promoter mutation has also been shown to further classify tumors into clinically-meaningful groups. The Cancer Genome Atlas (TCGA) recently identified six methylation subtypes of glioma. Thus, as part of a clinical transformation process, we evaluated sensitivity and specificity of the Illumina methylation array to classify adult gliomas into appropriate WHO groups. We molecularly profiled 86 specimens from our institution via Illumina EPIC methylation array: 76 adult gliomas, seven other brain tumors (anaplastic ependymoma, glial neoplasm, pilocytic astrocytoma) and three gliosis samples. The 76 adult gliomas also have Affymetrix OncoScan array, IDH1/2 and TERT promoter mutation sequencing data. Using only methylation data, the EPIC array correctly classified 70 (92%) of 76 gliomas into appropriate WHO groups. When EPIC array copy number data were used to verify codeletion status, 71 (93%) were correctly classified. Similarly, using only Illumina methylation data, 560 (90%) of 622 TCGA gliomas were correctly classified into WHO groups. When methylation array copy number data were used to verify codeletion status 611 (98%) were correctly classified. While the OncoScan and EPIC methylation array had 100% concordant codeletion status, there were discrepancies for other genomic regions; especially in gliomas that carry IDH1/2, ATRX and TP53 mutations. All seven (100%) of the other brain tumors were classified as LGm6. With respect to TERT mutation, of 44 TCGA IDH wildtype, TERT wild-type and non-codeleted gliomas, 64% were classified as LGm6; thus, supporting the importance of TERT for further classifying gliomas. Overall, when methylation and 1p/19q copy number data are used, the Illumina methylation array correctly classified most adult gliomas into appropriate WHO groups. However, caution should be used when evaluating genome-wide copy number alterations solely using the methylation array.