The authors have reviewed the treatment results in 42 patients with intracranial oligodendroglioma treated from 1940 through 1983 at the University of California, San Francisco. Two patients who died postoperatively were excluded from analysis. Eleven patients had mixed tumors, with a minor astrocytic component. The overall survival rates for the 29 patients with pure oligodendroglioma were 61% and 33% at 5 and 10 years, respectively; these rates for the 11 patients with mixed tumors were 57% and 38% at 5 and 10 years, respectively. The 10-year survival rate for 14 patients with pure oligodendroglioma who received greater than 45 Gy irradiation was 56% versus 18% for 11 patients who did not receive postoperative irradiation (p = 0.09). Nine patients with mixed tumor who received more than 45 Gy postoperatively had survival rates similar to those for the 14 patients with pure tumors irradiated with more than 45 Gy (p = 0.89). All patients who died of their tumor had evidence of intracranial recurrence. One patient, who did not receive initial postoperative irradiation, also had clinical and myelographic evidence of spinal seeding. All five patients examined postmortem had tumor recurrence at the primary site; one patient also had intraventricular seeding. Six of the 10 patients with pure oligodendroglioma who had a repeat biopsy at the time of tumor recurrence or at postmortem examination showed histological progression to an anaplastic astrocytoma or glioblastoma multiforme. Based on this study, adult patients with pure or mixed oligodendroglioma currently are treated with partial-brain irradiation to a dose of about 60 Gy. In general, children are treated with partial-brain irradiation to about 50 Gy.