The sonographic findings observed in four patients with primary retroperitoneal cysts were reviewed. All lesions presented as fluid-filled masses with anechoic content and regular margins; all increased through transmission. Preoperative location of the cysts within the retroperitoneal space was possible in all cases by either real-time ultrasound (US) analysis of the different relationships of the kidneys and the cysts during respiratory movements or the anterior displacement of surrounding structures. A differential diagnosis among retroperitoneal lesions containing fluid is difficult and cannot be obtained with imaging methods alone. However, preoperative knowledge of both the retroperitoneal location of the disease process and its fluid content may help to narrow the diagnostic possibilities, thus allowing the surgeon to more accurately plan the therapeutic approach to the patient.