Background/Methods: Gliomas are common malignant neoplasms of the central nervous system. Among the major subtypes of gliomas, oligodendrogliomas are distinguished by their remarkable sensitivity to chemotherapy, with approximately two thirds of anaplastic (malignant) oligodendrogliomas responding dramatically to combination treatment with procarbazine, lomustine, and vincristine (termed PCV). Unfortunately, no clinical or pathologic feature of these tumors allows accurate prediction of their response to chemotherapy. Anaplastic oligodendrogliomas also are distinguished by a unique constellation of molecular genetic alterations, including coincident loss of chromosomal arms 1p and 19q in 50%-70% of tumors. We have hypothesized that these or other specific genetic changes might predict the response to chemotherapy and prognosis in patients with anaplastic oligodendrogliomas. Therefore, we have analyzed molecular genetic alterations involving chromosomes 1p, 10q, and 19q and the TP53 (on chromosome 17p) and CDKN2A (on chromosome 9p) genes, in addition to clinicopathologic features in 39 patients with anaplastic oligodendrogliomas for whom chemotherapeutic response and survival could be assessed. Results/Conclusions: Allelic loss (or loss of heterozygosity) of chromosome 1p is a statistically significant predictor of chemosensitivity, and combined loss involving chromosomes 1p and 19q is statistically significantly associated with both chemosensitivity and longer recurrence-free survival after chemotherapy. Moreover, in both univariate and multivariate analyses, losses involving both chromosomes 1p and 19q were strongly associated with longer overall survival, whereas CDKN2A gene deletions and ring enhancement (i.e., contrast enhancement forming a rim around the tumor) on neuroimaging were associated with a significantly worse prognosis. The inverse relationship between CDKN2A gene deletions and losses of chromosomes 1p and 19q further implies that these differential clinical behaviors reflect two independent genetic subtypes of anaplastic oligodendroglioma. These results suggest that molecular genetic analysis may aid therapeutic decisions and predict outcome in patients with anaplastic oligodendrogliomas. [J Natl Cancer Inst 1998;90:1473-9]