BACKGROUND: Among number of genetic markers in gliomas, 1p/19q co-deletion in grade 2-3 gliomas is the most established marker in predicting chemosensitivity and surprisingly better prognosis. The mechanism for such distinct clinical features is not well understood. METHODS: We have retrospectively analyzed the treatment result of 1p/19q codeleted gliomas to elucidate the mechanism underlying such clinical features. For recurrent tumors that were resected, genetic and histological examinations were performed to see the changes in the recurrent tumors. RESULTS: Of 31 gliomas with 1p/19q co-deletion, 5 were not given any adjuvant therapy, 18 were treated with chemotherapy alone, and 7 underwent radiation therapy with chemotherapy. With the median follow-up of 48 months, 12 out of the 31 recurred, but the histological grades remained to be 2 or 3, showing slow growth, and responded to additional therapy well. In 9 cases, genetic testing for 1p/19q, 10q loss was peformed in recurrent tumors, and the alterations remained the same without additional 10q loss, the hallmark of grade 4 gliomas (glioblastoma). Copy number alterations of whole genome was also examined in selected cases, that also showed fewer additional genetic changes compared to the previous studies on astrocytic tumors. CONCLUSIONS: Thus, slow growth and relative genetic stability after genotoxic treatment may be the feature of those gliomas leading to the better prognosis compared to the rest. SECONDARY CATEGORY: Clinical Neuro-Oncology.