An 89-year-old man was admitted to the hospital due to intermittent anterior chest wall pain for > 1 month. A chest radiograph obtained on November 9, 2004, demonstrated a mass with an irregular border, inside a thin-walled cavity, located in the superior segment of the left lower lobe. A chest CT scan revealed an irregular thin-walled cavity, 5.9 × 5.4 × 4 cm in size, with an air-crescent sign in the superior segment of the left lower lobe, and an intracavitary fungus ball-like mass. A bronchoscopic examination was performed, revealing only external compression of the left lower lobe bronchial lumen. Cultures from both the brushing cytology and brushing fungus specimens were negative. Since the patient was a heavy smoker and the chest radiograph obtained 23 months before had revealed no active pulmonary lesion, neoplastic growth was still highly suspected. Thus, an 18F-fluoro-2-deoxyglucose positron emission tomography study was performed on November 25, and a mass with a slightly increased standard uptake value (3.17; cutoff value, 2.5) was found. He received a left lower lobe lobectomy on December 23, and a tumor with many septum-like structures connecting the surrounding pulmonary parenchymal tissue was found in the superior segment of the left lower lobe. The final pathologic diagnosis was adenocarcinoma of the lung (pT2N0M0). Thus, even though the chest radiograph and chest CT scan showed a typical air-crescent sign (ie, mass inside a cavity) favoring a mycetoma, the physician should still keep in mind that lung cancer may also unusually present in this way.