BACKGROUND: This study evaluated the association between health-related quality of life (HRQOL) and cognition in patients receiving memantine for prevention of cognitive dysfunction during whole-brain radiotherapy (WBRT). METHODS: Adult patients with brain metastases received WBRT and were randomized to receive placebo or memantine, 20 mg per day, within 3 days of initiating radiotherapy, for 24 weeks. The Functional Assessment of Cancer Therapy-Brain module (FACT-Br) and Medical Outcomes Scale-Cognitive Functioning Scale (MOS-C) were completed in coordination with serial standardized tests of cognitive function. RESULTS: Of the 508 eligible patients, 442 (87%) consented to participate in the HRQOL portion and contributed to baseline analyses. Evaluable patients at 24 weeks (n = 246) included surviving patients completing FACT-Br, MOS-C, and objective cognitive assessments (n = 146, 59%) and patients alive at time of missed assessment (n = 100, 41%). Baseline cognitive function correlated significantly with FACT-Br and MOS-C self-reports. All domains of objective cognitive function showed declines over time. Neither FACT-Br nor MOS-C differed between the treatment arms. Emotional and functional well-being subscales of the FACT improved over time while the remainder of the FACT-Br domains remained stable. MOS-C scores declined over time. CONCLUSION: Baseline cognitive function correlated significantly with FACT-Br and MOS-C scores. No by-arm differences in HRQOL were observed despite differences in objective cognitive function. Patient attrition and poor testing compliance remain significant problems in studies of cognitive function of brain metastases patients and further effort is needed to improve compliance with testing and sensitivity of patient-reported measures.