Oligodendroglial tumors frequently have deletions ofchromosomal loci on 1p and 19q. Loss of heterozygosity (LOH) of chromosome 10 may be a negative prognostic factor. We reviewed 23 patients with oligodendroglial tumors, to evaluate the frequency of 1p and 10q LOH and correlate with clinical outcome. Three loci (D1S402, D1S1172, MCT118) on 1p and 2 loci (D10S520 and D10S521) on 10q were analyzed for LOH using PCR techniques. Sixteen oligodendrogliomas (6 low grade and 10 anaplastic) and 7 oligoastrocytomas (1 low grade and 6 anaplastic) were studied. Overall 14/22 (64%) showed 1p LOH and 7/23 (30%) showed 10q LOH. Of 7 patients with some response to chemotherapy, all showed 1p LOH and none had 10q LOH. Of 5 patients with stable or progressive disease, 1 had 1p LOH and 2 showed 10q LOH. The presence of 1p LOH was significantly associated with response to chemotherapy (p = 0.02). Median progression free survival (PFS) was 31 months for 1p intact patients and 118 months for the 1p LOH group (p = 0.014). Median PFS for 10q LOH patients was 31 and 118 months for patients with intact chromosome 10 (p = 0.016).1p LOH is a predictor of response to chemotherapy and a good prognostic factor. 10q LOH is less common in oligodendroglial tumors but predicts for worse outcome. Molecular genotyping of oligodendroglial tumors is recommended as part of the standard diagnostic workup.