OBJECTIVE The optimal adjuvant management for atypical meningiomas remains controversial. The aim of this study was to review long-term outcomes to identify potential prognostic factors for disease progression. METHODS From August 1992 to August 2013, 70 patients with atypical meningioma were treated at the authors' institution. Pathology revision was performed based on WHO 2007 criteria. Patients with multiple tumors, neurofibromatosis Type 2, or inadequate imaging follow-up were not eligible. The authors performed pre- and postoperative serial measurements of tumor volume from MRI. Age, sex, tumor location, bone involvement, brain invasion, mitotic figures, preoperative disease volume, extent of resection, tumor growth rates, use of adjuvant postoperative radiation therapy (PORT), and residual tumor volume at the time of radiation therapy (RT) were assessed by univariate and multivariate analysis to determine their potential impact on disease progression. RESULTS Forty patients (57%) underwent gross-total resection (GTR) and 30 (43%) underwent subtotal resection (STR). PORT was delivered to 12 patients (30%) with a GTR and in only 4 (13%) with an STR. The 5-year progression-free survival (PFS) rate for patients in the GTR group with or without PORT was 100% and 54.1%, respectively (p = 0.0058). PFS for patients in the STR group with or without PORT was 75% and 0%, respectively (p = 0.0026). On multivariate analysis, STR and PORT were the only independent significant prognostic factors for disease progression with hazard ratios of 5.4873 (95% CI 2.19-13.72, p = 0.0003) and 0.0464 (95% CI 0.0059-0.364, p = 0.0035), respectively. Based on Youden's index statistic, a cutoff residual tumor volume of more than 8.76 cm3 at the time of RT was associated with worse PFS (13.6% vs 56%, p = 0.0079). Before receiving RT, the median relative and absolute growth rates and tumor doubling time for patients were 124.2%/year, 4.8 cm3/year, and 1.67 years, respectively. These indices changed after RT to 0.245%/year, -0.09 cm3/year, and -0.005 year, respectively (p < 0.05). CONCLUSIONS In atypical meningioma, the use of PORT is associated with improved PFS even in patients who undergo GTR. Patients with residual tumor volume larger than 8.76 cm3 have an increased risk of disease progression and should be considered for early RT.