We report a rare esophageal duplication cyst, in a 12-year-old girl. The cyst had enlarged rapidly within 2 years. In December 1997, on admission, computed tomography and magnetic resonance imaging demonstrated a cystic mass in the pleural cavity. We resected the cyst and the adjacent lung. Histopathological examination revealed an esophageal duplication cyst. Her presenting symptoms of fever and cough may have been related to infection of the cyst.