PURPOSE: The rate of missing data on patient-reported health-related quality of life (HRQOL) in brain tumor clinical trials is particularly high over time. One solution to this issue is the use of proxy (i.e., partner, relative, informal caregiver) ratings in lieu of patient-reported outcomes (PROs). In this study we investigated patient-proxy agreement on HRQOL outcomes in high-grade glioma (HGG) patients. METHODS: Generic and disease-specific HRQOL were assessed using the EORTC QLQ-C30 and QLQ-BN20 in a sample of 501 patient-proxy dyads participating in EORTC trials 26101 and 26091. Patients were classified as impaired or intact, based on their neurocognitive performance. The level of patient-proxy agreement was measured using Lin's concordance correlation coefficient (CCC) and the Bland-Altman limit of agreement. The Wilcoxon signed-rank test was used to evaluate differences between patients' and proxies' HRQOL. RESULTS: Patient-proxy agreement in all HGG patients (N = 501) ranged from 0.082 to 0.460. Only 18.8% of all patients were neurocognitively intact. Lin's CCC ranged from 0.088 to 0.455 in cognitively impaired patients and their proxies and from 0.027 to 0.538 in cognitively intact patients and their proxies. CONCLUSION: While patient-proxy agreement on health-related quality of life outcomes is somewhat higher in cognitively intact patients, agreement in high-grade glioma patients is low in general. In light of these findings, we suggest to cautiously consider the use of proxy's evaluation in lieu of patient-reported outcomes, regardless of patient's neurocognitive status.