Mutations in isocitrate dehydrogenase 1 (IDH1) are found in a high proportion of glial tumors and have a significant prognostic impact. Although direct sequencing has been considered to be the gold-standard method to detect this mutation, the sensitivity of this technique has been questioned especially because specimens from glial tumors may contain large numbers of non-tumor cells. We screened 141 cases of oligodendroglial tumors for IDH1 mutations using peptide nucleic acid (PNA)-mediated clamping polymerase chain reaction (PCR) and compared the results with the results of direct sequencing, pyrosequencing, and immunohistochemistry (IHC). Nested PCR was only performed in cases having mutant IDH1 only discovered by clamping PCR. Using dilution experiments mixing IDH1 wild-type and mutant DNA samples, clamping PCR detected mutations in samples with a 1% tumor DNA composition. Using PNA clamping PCR, we detected 138 of 141 (97.9%) cases with mutant IDH1 in our series, which is significantly higher (P = 0.016; PNA clamping vs. direct sequencing) than those of direct sequencing (74.5%), pyrosequencing (75.2%) and IHC (75.9%). From our results, almost all oligodendroglial tumors have IDH1 mutations, and this suggests that IDH1 mutation is an early and common event especially in the development of oligodendroglial tumors.