Glioblastoma is the most malignant and frequent of the glial tumors. A minor fraction of glioblastoma may contain areas showing oligodendroglioma-like tumor cell differentiation. Several authors have described such tumors as glioblastoma with oligodendroglial component (GBMO). GBMO may represent the ultimate level of malignancy in the oligodendroglial lineage. The oligodendroglial component and combined loss of chromosomal arm 1p and 19q in glioblastoma indicate increased survival. In our study, we analyzed 1p and 19q status in a series of 12 glioblastoma and 8 oligodendroglial tumors using fluorescence in situ hybridization (FISH) on paraffin-embedded tissues. In each case, hybridization status was classified as deletion, imbalance, polysomy, amplification, or normal pattern. Other genetic alterations such as CDKN2A (p16), RB, and EGFR were also assessed. On histological review, 2 of 12 glioblastoma (16.7%) were classified as GBMO. Chromosome 1p/19q deletion was detected in 3 of 12 glioblastomas (25%). In contrast, all 8 oligodendroglial tumors showed 1p/19q deletion. All GBMO had 19q deletion with imbalance, whereas 1 of 10 ordinary glioblastoma (10%) demonstrated 19q deletion with imbalance. All but 1 ordinary glioblastoma (90%) showed CDKN2A (p16) deletion, but no GBMO displayed this alteration. Our results indicate that GBMO may be a distinct subtype of glioblastoma harboring a characteristic molecular profile. FISH on paraffin-embedded specimens is a useful method for subclassification of glioblastoma.