A 46-year-old woman presented with a 1-year history of progressive left-arm numbness. A cyst below the left hemidiaphragm was discovered incidentally when a CT scan was performed to examine the thymus for a suspected tumor. A thymic mass was found. MRI indicated that the cyst contained proteinaceous fluid. The thymoma was approached through a median sternotomy and resected, but the cyst was found to be infradiaphragmatic. A separate, left paraspinal incision was made to access the retroperitoneum, and the cyst was resected. Histologic examination showed that the cyst was bronchogenic in origin. Retroperitoneal bronchogenic cysts are very rare, and only four cases have been reported in the English-language literature.