BACKGROUND The determinants of outcomes for adult survivors of pediatric low‐grade glioma (PLGG) are largely unknown. METHODS This study collected population‐based follow‐up information for all PLGG patients diagnosed in Ontario, Canada from 1985 to 2012 (n = 1202) and determined factors affecting survival. The impact of upfront radiation treatment on overall survival (OS) was determined for a cohort of Ontario patients and an independent reference cohort from the Surveillance, Epidemiology, and End Results database. RESULTS At a median follow‐up of 12.73 years (range, 0.02‐33 years), only 93 deaths (7.7%) were recorded, and the 20‐year OS rate was 90.1% ± 1.1%. Children with neurofibromatosis type 1 had excellent survival and no tumor‐related deaths during adulthood. Adverse risk factors included pleomorphic xanthoastrocytoma (P < .001) and a thalamic location (P < .001). For patients with unresectable tumors surviving more than 5 years after the diagnosis, upfront radiotherapy was associated with an approximately 3‐fold increased risk of overall late deaths (hazard ratio HR, 3.3; 95% confidence interval CI, 1.6‐6.6; P = .001) and an approximately 4‐fold increased risk of tumor‐related deaths (HR, 4.4; 95% CI, 1.3‐14.6; P = .013). In a multivariate analysis, radiotherapy was the most significant factor associated with late all‐cause deaths (HR, 3.0; 95% CI, 1.3‐7.0; P = .012) and tumor‐related deaths (HR, 4.4; 95% CI, 1.3‐14.6; P = 0.014). A similar association between radiotherapy and late deaths was observed in the independent reference cohort (P < .001). In contrast to early deaths, late mortality was associated not with PLGG progression but rather with tumor transformation and non‐oncological causes. CONCLUSIONS The course of PLGG is associated with excellent long‐term survival, but this is hampered by increased delayed mortality in patients receiving upfront radiotherapy. These observations should be considered when treatment options are being weighed for these patients. Cancer 2016;122:1261–9. © 2016 American Cancer Society. This is a large population‐based study looking at long‐term survivors of childhood low‐grade glioma. Although the course of pediatric low‐grade glioma is associated with excellent long‐term survival, this study unravels worrisome information about late deaths for patients receiving upfront radiation therapy.