Morphologic criteria for diagnosing oligodendrogliomas and for classifying them as well-differentiated (World Health Organization grade II) and anaplastic (World Health Organization grade III) are well recognized. Nevertheless, applying these guidelines to speci c cases often reveals discrepancies among different observers. In addition, whether a given tumor also contains an astrocytic component may be debatable. Loss of heterozygosity studies have demonstrated that oligodendroglial neoplasms have a high incidence of loss of the 1p and 19q chromosomal arms. Although loss of heterozygosity for portions of 19q are sometimes seen in astrocytic neoplasms, these tumors seldom show complete loss of 19q accompanied by loss of 1p. Loss of 9p or homozygous deletion of the CDKN2 gene or both are associated with anaplastic oligodendrogliomas, whereas loss of 17p or TP53 gene mutations or both are frequent in astrocytomas, but rare in oligodendrogliomas. These observations suggest that molecular genetic parameters could provide an objective, reproducible framework for classifying oligodendroglial neoplasms. Neuro-Oncology 1, 52-60, 1999 (Posted to Neuro-Oncology [serial online], Doc. 98-20, January 19, 1999 O ligodendrogliomas were rst described as a speci c histologic type of glial neoplasm by Bailey and colleagues (Bailey and Bucy, 1929;Bailey and Cushing, 1926). Since the original description, these tumors have been recognized as often containing a mixture of astrocytic and oligodendroglial elements. In addition, anaplastic oligodendrogliomas, which display histologic features such as cellular atypia, increased cellular density, endothelial proliferation, and necrosis, often behave more aggressively than the slowgrowing, well-differentiated tumors. The WHO 3 classication scheme for brain tumors sets forth criteria for distinguishing between grades II and III oligodendrogliomas and mixed gliomas. Nevertheless, application of these criteria to individual cases often reveals a lack of consistency and reproducibility in classifying oligodendroglial tumors among observers.Objective means of determining whether a tumor contains neoplastic oligodendroglial or astrocytic elements or both and distinguishing between grades II and III lesions are critical because the survival and therapy is different for each category of tumor. A subset of oligodendroglial tumors characterized by loss of chromosomal arms 1p and 19q suggests that demonstration of this pattern by LOH, FISH, or CGH studies might provide an objective, reproducible means of identifying this category of tumors. Furthermore, LOH for 9p or deletions of the CDKN2 gene or both appear to occur more frequently in anaplastic oligodendrogliomas than in well-differentiated, grade II tumors. Maintz et al. (1997) observed that though oligodendroglial tumors have frequent loss of 1p and 19q and a low incidence of TP53 gene mutations, astrocytomas have the opposite pro le. They evaluated these parameters in a series of gliomas, which included 38 oligoastrocytomas of grades II and ...