Background: We evaluated vorasidenib (VOR), a dual inhibitor of mIDH1/2, in patients with mIDH1/2 glioma (Phase 3; NCT04164901). Methods: Patients with residual/recurrent grade 2 mIDH1/2 oligodendroglioma or astrocytoma were enrolled (age ≥12; Karnofsky Performance Score ≥80; measurable non-enhancing disease; surgery as only prior treatment; not in immediate need of chemoradiotherapy). Patients were stratified by 1p19q status and baseline tumor size and randomized 1:1 to VOR 40 mg or placebo (PBO) daily in 28-day cycles. Endpoints included imaging-based progression-free survival (PFS), time to next intervention (TTNI), tumor growth rate (TGR), health-related quality of life (HRQoL), neurocognition and seizure activity. Results: 331 patients were randomized (VOR, 168; PBO, 163). The median age was 40.0 years. 172 and 159 patients had histologically confirmed oligodendroglioma and astrocytoma, respectively. Treatment with VOR significantly improved PFS and TTNI. Median PFS: VOR, 27.7 mos; PBO, 11.1 mos (P=0.000000067). Median TTNI: VOR, not reached; PBO, 17.8 mos (P=0.000000019). Treatment with VOR resulted in shrinkage of tumor volume. Post-treatment TGR: VOR, -2.5% (95% CI: -4.7, -0.2); PBO, 13.9% (95% CI: 11.1, 16.8). HRQoL and neurocognition were preserved and seizure control was maintained. VOR had a manageable safety profile. Conclusions: VOR was effective in mIDH1/2 diffuse glioma not in immediate need of chemoradiotherapy.