We report a rare case of thoracoabdominal intestinal duplication with absent inferior vena cava (IVC). The patient was initially explored with a mistaken diagnosis of diaphragmatic hernia on the basis of a chest radiograph and barium meal. However, a subsequent computed tomography scan revealed a mediastinal mass with an air-fluid level, a hugely dilated azygos vein, and an absent IVC. Thoracoabdominal exploration was required to excise the duplication cyst arising from the jejunum. We believe that this is the first report of this association.