Isolated retroperitoneal bronchogenic cysts are extremely rare. We report a case which was intradiaphragmatic intimately associated with the musculature of the left crus diaphragmatic and unconnected with any other structures. Ultrasound and computed tomography findings were consistent with a pancreatic or an adrenal mass. Pathology confirmed a bronchogenic cyst. The outcome is favourable and the overall prognosis is good.