A 70-year-old woman with right lung cancer was admitted to our hospital. Chest computed tomography (CT) revealed an approximate 2.5 cm sized mass in the right middle lobe, and enlarged hilar and mediastinal lymph nodes. The 2-fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET) showed high uptake in the right hilar and mediastinal nodes. Therefore we diagnosed N2 (mediastinal nodal involvement) disease. Microscopically, however, all dissected lymph nodes revealed anthracosilicosis and negative for malignancy. Although false-positive evaluation of mediastinal involvement by PET can occur in the setting of metabolically active inflammatory disease, this case did not have these active diseases. The FDG-PET result of this case was unusual.