IMPORTANCE: A recent randomized radiation dose-escalation trial in unresectable stage III non-small-cell lung cancer (NSCLC) (Radiation Therapy Oncology Group [RTOG] 0617) showed a lower survival rate in the high-dose radiation therapy (RT) arm (74 Gy) than in the low-dose arm (60 Gy) with concurrent chemotherapy. OBJECTIVE: The primary QOL hypothesis predicted a clinically meaningful decline in quality of life (QOL) via the Functional Assessment of Cancer Therapy (FACT)-Lung Cancer Subscale (LCS) in the high-dose RT arm at 3 months. DESIGN, SETTING, AND PATIENTS: The RTOG 0617 trial was a randomized phase 3 study (conducted from November 2007 to November 2011) in stage III NSCLC using a 2 × 2 factorial design and stratified by histology, positron emission tomography staging, performance status, and irradiation technique (3-dimensional conformal RT [3D-CRT] vs intensity modulated RT [IMRT]). A total of 185 institutions in the United States and Canada took part. Of 424 eligible patients with stage III NSCLC randomized, 360 (85%) consented to QOL evaluation, of whom 313 (88%) completed baseline QOL assessments. INTERVENTION: Treatment with 74-Gy vs 60-Gy RT with concurrent and consolidation carboplatin/paclitaxel with or without cetuximab. MAIN OUTCOMES AND MEASURES: The QOL data were collected prospectively via FACT Trial Outcome Index (FACT-TOI), calculated as the sum of the following measures: Physical Well Being (PWB), Functional Well Being (FWB), and the LCS. Data are presented at baseline and 3 and 12 months via minimal clinically meaningful changes of 2 points or more for PWB, FWB, and LCS or 5 points or more for TOI. RESULTS: Of the 313 patients who completed baseline QOL assessments, 219 patients (70%) completed the 3-month QOL assessments, and 137 of the living patients (57%) completed the 12-month assessment. Patient demographics and baseline QOL scores were comparable between the 74-Gy and 60-Gy arms. Significantly more patients in the 74-Gy arm than in the 60-Gy arm had clinically meaningful decline in FACT-LCS at 3 months (45% vs 30%; P =.02). At 12 months, fewer patients who received IMRT (vs 3D-CRT) had clinically meaningful decline in FACT-LCS (21% vs 46%; P =.003). Baseline FACT-TOI was associated with overall survival in multivariate analysis. CONCLUSIONS AND RELEVANCE: Despite few differences in clinician-reported toxic effects between treatment arms, QOL analysis demonstrated a clinically meaningful decline in QOL in the 74-Gy arm at 3 months, confirming the primary QOL hypothesis. Baseline QOL was an independent prognostic factor for survival. TRIAL REGISTRATION: clinicaltrials.gov Identifier: NCT00533949.