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Background The optimal radiation dose for adult supratentorial low-grade glioma is unknown. The aim of this study was to provide a final update on oncologic and cognitive outcomes of high-dose versus low-dose radiation for low-grade glioma. Methods Between 1986 and 1994, 203 patients with supratentorial low-grade glioma were randomized (1:1) to 50.4 Gy in 28 fractions versus 64.8 Gy in 36 fractions after any degree of resection. Results For all patients, median overall survival (OS) was 8.4 years (95% CI: 7.2–10.8). Median progression-free survival (PFS) was 5.2 years (95% CI: 4.3–6.6). Median follow-up is 17.2 years for the 33 patients still alive. High-dose radiation did not improve 15-year OS (22.4%) versus low-dose radiation (24.9%, log-rank P = 0.978) or 15-year PFS (high dose, 15.2% vs low dose, 9.5%; P = 0.7142). OS was significantly better for patients with preoperative tumor diameter 27 and who underwent gross total resection. PFS was improved for patients with oligodendroglioma versus astrocytoma, preoperative tumor diameter 27. For patients who had normal MMSE at baseline, at 7 years only 1 patient (5%) had a clinically significant decrease in MMSE from the previous time point, with the remainder (95%) stable. None had decrease in MMSE at 10, 12, or 15 years. Conclusions Long-term follow-up indicates no benefit to high-dose over low-dose radiation for low-grade gliomas. Cognitive function appeared to be stable after radiation as measured by MMSE.