Bronchogenic cysts are benign congenital malformations usually. A retroperitoneal location is extremely uncommon. We reported a case of bronchogenic cyst occurred in the retroperitoneum in a 60-year-old patient. An abdominal CT for a prostatic adenocarcinoma staging has reported this incidental lesion. Biological, radiological and histological assessment confirmed the bronchogenic cyst diagnosis. The treatment of choice for most authors is surgical excision. However, because of a low malignant transformation risk, we have chosen a follow-up with abdominal computerized tomography every 6 months. During 3 years, biological and radiological assessments have shown a stable lesion. Therefore, it seems reasonable to propose a simple radiological monitoring for bronchogenic cysts in selected patients.