A 45-DAY-OLD, full-term, male infant weighing 3420 g at birth was admitted to the hospital with a 2-day history of vomiting and a clinical diagnosis of pyloric stenosis. He had been asymptomatic until the presenting complaint. Findings from radiological examination of the thorax revealed the presence of a large lucent cyst in the left upper lobe with mild deviation of the mediastinal structures (Figure 1). Routine laboratory analysis findings were normal. Pyloroplasty was performed on the third hospital day. The patient was discharged from the hospital 4 days after surgery and readmitted 1 week after discharge. A computed tomographic scan of the thorax showed a large cystic lesion with smooth borders in the upper lobe of the left lung. A lobectomy was performed.