The authors describe a rare case of mucoepidermal carcinoma of the lung incidentally identified in preoperative assessments for inguinal hernia repair in a 5-year-old girl. This patient was referred for right external inguinal hernia, and a 3.0-cm round-shaped lesion was found in the right lower lung field of a chest x-ray film. She had no respiratory tract complaints, but her serum carcinoembryonic antigen concentration was markedly elevated (21.2 ng/mL). Chest and abdominal computed tomography/magnetic resonance images could not determine the nature of the lesion, but 2-[18 F]fluoro-2-deoxy- d -glucose positron emission tomography (FDG-PET) indicated a malignant tumor pattern. The patient underwent a computed tomography-guided needle biopsy of the lesion (S8), which was soon followed by a right lower pulmonary lobectomy. Histopathology of the resected specimen showed mucoepidermal carcinoma with no regional lymph node metastasis. In childhood asymptomatic pulmonary lesions, it is often difficult to rule out the possibility of malignancy. In the present case, FDG-PET scanning appropriately indicated the therapeutic priority of pediatric thoracic surgery.