Background: The prognostic value of ¹⁸F-fluorodeoxyglucose (FDG) positron emission tomography-computed tomography (PET-CT) response assessment following first-line immunochemotherapy for advanced-stage symptomatic follicular lymphoma (FL) was previously demonstrated for patients (pts) enrolled in the Phase III GALLIUM study (NCT01332968; Trotman et al. ICML 2017). Here, we evaluated the association between PET complete metabolic response (CMR) and survival after longer follow-up in this patient population. Methods: In the GALLIUM study, 1202 pts with previously untreated FL were randomized 1:1 to induction therapy of 1000mg obinutuzumab (G; Days 1, 8, 15 of Cycle 1 then Day 1 of subsequent cycles) or 375mg/m² rituximab (R; Day 1 of each cycle), in combination with chemotherapy (CHOP, CVP, or bendamustine) (Marcus et al. New Engl J Med 2017). PET-CT scans were mandatory, where available, at baseline and end-of-induction (EOI) for the first 170 pts enrolled, and optional thereafter. For this response analysis, the Lugano 2014 criteria were applied by an independent review committee (IRC) (Cheson et al. J Clin Oncol 2014). Associations between EOI PET complete metabolic response (PET-CMR) status and progression-free survival (PFS) and overall survival (OS) were evaluated, with hazard ratios (HR) stratified according to chemotherapy regimen and FL International Prognostic Index. Results: Of the 609 pts with a baseline PET scan, 595 (98%) had detectable lesions. Of these, 519 pts had an EOI PET evaluable by Lugano 2014 criteria. At EOI, per IRC assessment, 450/595 (76%) pts had achieved CMR. Pts with non-available scans were considered as non-responders and were excluded from the landmark (LM) analyses. Pts who died or progressed (CT-based progression assessment) before or at EOI were excluded from the PFS LM analysis; pts who died before EOI were excluded from the OS LM analysis. After a median follow-up of 76.5 months, EOI PET status was highly prognostic for both longer investigator-assessed PFS (non-CMR vs CMR: HR 3.40; 95% CI: 2.33–4.97; p < 0.0001) and longer OS (HR 3.34; 95% CI: 1.81–6.17; p < 0.0001). Six-year investigator-assessed PFS from EOI was 62.6% (95% CI: 57.0–67.6) for CMR pts compared with 23.4% (95% CI: 12.2–36.7) for non-CMR pts; the corresponding OS was 91.3% (95% CI: 88.1–93.6) vs 79.6% (95% CI: 68.0–87.4). Conclusions: With more than 6 years of follow-up, this analysis confirms that after first-line chemoimmunotherapy for FL, achieving CMR on PET-CT is an early and strong predictor of increased PFS and OS. Clinical trial information: NCT01332968.