A 55-year-old man presented with progressive pain on the left hemithorax. A mass in the left upper lobe was seen on the chest radiograph. CT showed mediastinal lymphadenopathy and a mass in the left upper lobe with invasion of the chest wall. The results of all cytologic and microbiologic examinations were normal. The clinical diagnosis was bronchial carcinoma. A PET scan showed enhanced uptake within the left chest wall without mediastinal lymphadenopathy. At that stage, the left hemithorax had become indurated. Microbiologic examination of pus, which dripped out of the mass after incision, showed gram-positive anaerobic organisms. A clinical diagnosis of actinomycosis was made.