© 2016 by American Society of Clinical Oncology. Purpose: Posterior fossa ependymoma comprises two distinct molecular variants termed EPN-PFA and EPN-PFB that have a distinct biology and natural history. The therapeutic value of cytoreductive surgery and radiation therapy for posterior fossa ependymoma after accounting for molecular subgroup is not known. Methods: Four independent nonoverlapping retrospective cohorts of posterior fossa ependymomas (n = 820) were profiled using genome-wide methylation arrays. Risk stratification models were designed based on known clinical and newly described molecular biomarkers identified by multivariable Cox proportional hazards analyses. Results: Molecular subgroup is a powerful independent predictor of outcome even when accounting for age or treatment regimen. Incompletely resected EPN-PFA ependymomas have a dismal prognosis, with a 5-year progression-free survival ranging from 26.1% to 56.8% across all four cohorts. Although first-line (adjuvant) radiation is clearly beneficial for completely resected EPN-PFA, a substantial proportion of patients with EPN-PFB can be cured with surgery alone, and patients with relapsed EPN-PFB can often be treated successfully with delayed external-beam irradiation. Conclusion: The most impactful biomarker for posterior fossa ependymoma is molecular subgroup affiliation, independent of other demographic or treatment variables. However, both EPN-PFA and EPN-PFB still benefit from increased extent of resection, with the survival rates being particularly poor for subtotally resected EPN-PFA, even with adjuvant radiation therapy. Patients with EPN-PFB who undergo gross total resection are at lower risk for relapse and should be considered for inclusion in a randomized clinical trial of observation alone with radiation reserved for those who experience recurrence.