Correct staging of lung cancer is important for the selection of the best therapy, but discriminating between lymphadenopathy from lung cancer and from sarcoidosis by imaging examinations is difficult. Additionally, distinguishing lymphadenopathy of sarcoidosis from sarcoid reactions which are sometimes caused by lung cancer is difficult on imaging and pathological findings. A 73-year-old woman was diagnosed as lung cancer clinical T1bN3M0 stage ШB based on false-positive 18F-fluoro-2-deoxyglucose positron emission tomography uptake. Because the effects of chemotherapy were different between the lymphadenopathy and the primary lesion, endobronchial ultrasound-guided transbronchial needle aspiration (EBUS-TBNA) was performed and revealed sarcoidosis as the cause of the lymphadenopathy with using a specific monoclonal antibody against Propionibacterium acnes (PAB antibody). Accordingly, the stage was changed to clinical T1bN0M0 stage ІA, for which radical operation was performed. EBUS-TBNA should be performed aggressively when the effect of chemotherapy is different between lymphadenopathies and other lesions, and the PAB antibody can help to discriminate between sarcoidosis and sarcoid reactions caused by lung cancer. The combination of EBUS-TBNA and the PAB antibody is expected to be valuable in the definitive diagnosis of a lymphadenopathy for the staging of lung cancer.