A case of idiopathic retroperitoneal fibrosis is reported with a description of the gray scale ultrasound findings. The mass was indistinguishable in appearance from many other retroperitoneal processes, such as lymphoma, fibrosarcoma, and liposarcoma. In this case the mass had a well-defined anterior border, and the posterior border with the vena cava and aorta was difficult to demonstrate. The sound transmission properties of the mass were similar to those of normal soft tissue structures. The mass, which showed no echoes at moderate gain levels, produced weak echoes at high gain levels. By marking the projection of the tumor as seen by ultrasound on the skin and then obtaining an x ray with a lead wire on these marks, a comparison of ultrasound with other studies is possible. This also helps in demonstrating the location and extent of the mass. Ultrasound is suggested as a useful noninvasive technique for following the progress of retroperitoneal fibrosis.