Objective: The presence of pathological necrosis in the tumor is known to be a factor indicating worse survival. Our study defined necrosis in staging 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) in patients with stage IIIB non-small-cell lung cancer (NSCLC) to investigate whether this is a poor prognostic marker. Methodology: A total of 77 patients with NSCLC were evaluated. To evaluate necrosis on 18F FDG PET/CT, we drew a region of interest (ROI) in the area showing visually very low/or no FDG uptake on PET and PET/CT fusion images. If SUVmax was less than blood pool SUVmax and showed significantly less attenuation [10 to 30 Hounsfield units (HUs)] than surrounding tissue on low-dose correlative CT with non-intravenous contrast, we defined it as necrotic (PETNECROSIS). We evaluated the relationship of SUVmax, tumor size, and PETNECROSIS with progression-free survival (PFS) using a Cox proportional hazard regression model. Results: A PFS analysis was performed on 16 patients treated with standard chemoradiotherapy (CRT) regimen. Tumor size <_42 mm versus > 42 mm (P = 0.044, HR: 6.103, 95 CI%: 1.053-35.358) and PETNECROSIS presence/absence (P = 0.027, HR: 6.719, 95 CI%: 1.245-36.264) were independent predictors for PFS. Patients with tumor size <_42 mm and PET(NECROSIS )absence were associated with higher 1-year PFS rate than patients with tumor size > 42 mm and PETNECROSIS presence (86% vs. 63.5% P = 0.005 and 87.5% vs. 29%, P = 0.001, respectively). Conclusion: PETNECROSIS is helpful to distinguish the patients who would suffer worse survival in stage IIIB NSCLC.