Gliosarcoma, a biphasic tumor with both mesenchymal and glial elements, is typically considered a variant of astrocytoma (glioblastoma), WHO Grade IV. A 57-year-old man presented with altered mental status and was found to have a large right frontal mass. Biopsy and subsequent subtotal resection revealed a WHO Grade II oligodendroglioma with classic histological features, expression of IDH1 R132H mutant protein, and chromosome 1p19q co-deletion. Fifteen months later, the patient developed recurrent tumor composed of intersecting fascicles of spindled cells with necrosis and a high mitotic index. The recurrent tumor stained for both mesenchymal and glial elements, consistent with the diagnosis of gliosarcoma, and showed retained IDH1 R132H expression. By FISH analysis, the gliosarcoma showed no evidence of 1p19q co-deletion. We performed SNP arrays and detailed SNP analysis of both the oligodendroglioma and the gliosarcoma. This demonstrated loss of heterozygosity (LOH) of chromosomes 1 and 19 in the gliosarcoma with retention of the same full-length chromosomes 1 and 19 found intact in the oligodendroglioma. Not surprisingly, the gliosarcoma harbored multiple additional alterations, consistent with clonal evolution. There have been only rare reports of sarcomatous transformation of oligodendroglioma ("oligosarcoma") and most were published prior to the development of modern genetic modalities. Here we present a case with detailed genetic evidence that suggests that mesenchymal metaplasia sarcomatous transformation is possible in classic oligodendrogliomas with 1p19q codeletions.