OBJECTIVE WHO Grade III gliomas are relatively rare and treated with multiple modalities such as surgery, chemotherapy, and radiotherapy. The impact of the extent of resection (EOR) on improving survival in patients with this tumor type is unclear. Moreover, because of the heterogeneous radiological appearance of Grade III gliomas, the MRI sequence that best correlates with tumor volume is unknown. In the present retrospective study, the authors evaluated the prognostic significance of EOR. METHODS Clinical and radiological data from 122 patients with newly diagnosed WHO Grade III gliomas who had undergone intraoperative MRI-guided resection at a single institution between March 2000 and December 2011 were analyzed retrospectively. Patients were divided into 2 groups by histological subtype: 81 patients had anaplastic astrocytoma (AA) or anaplastic oligoastrocytoma (AOA), and 41 patients had anaplastic oligodendroglioma (AO). EOR was calculated using pre- and postoperative T2-weighted and contrast-enhanced T1-weighted MR images. Univariate and multivariate analyses were performed to evaluate the prognostic significance of EOR on overall survival (OS). RESULTS The 5-, 8-, and 10-year OS rates for all patients were 74.28%, 70.59%, and 65.88%, respectively. The 5- and 8-year OS rates for patients with AA and AOA were 72.2% and 67.2%, respectively, and the 10-year OS rate was 62.0%. On the other hand, the 5- and 8-year OS rates for patients with AO were 79.0% and 79.0%; the 10-year OS rate is not yet available. The median pre- and postoperative T2-weighted high-signal intensity volumes were 56.1 cm (range 1.3-268 cm) and 5.9 cm (range 0-180 cm), respectively. The median EOR of T2-weighted high-signal intensity lesions (T2-EOR) and contrast-enhanced T1-weighted lesions were 88.8% (range 0.3%-100%) and 100% (range 34.0%-100%), respectively. A significant survival advantage was associated with resection of 53% or more of the preoperative T2-weighted high-signal intensity volume in patients with AA and AOA, but not in patients with AO. Univariate analysis showed that preoperative Karnofsky Performance Scale score (p = 0.0019), isocitrate dehydrogenase 1 ( IDH1) mutation (p = 0.0008), and T2-EOR (p = 0.0208) were significant prognostic factors for survival in patients with AA and AOA. Multivariate analysis demonstrated that T2-EOR (HR 3.28; 95% CI 1.22-8.81; p = 0.0192) and IDH1 mutation (HR 3.90; 95% CI 1.53-10.75; p = 0.0044) were predictive of survival in patients with AA and AOA. CONCLUSIONS T2-EOR was one of the most important prognostic factors for patients with AA and AOA. A significant survival advantage was associated with resection of 53% or more of the preoperative T2-weighted high-signal intensity volume in patients with AA and AOA.