We report the findings on computed tomography (CT) in idiopathic and secondary retroperitoneal fibrosis in 7 patients. In all cases CT showed the fibrotic mass, accurately defining its extent and the involvement of adjacent anatomic structures. The densitometric and anatomopathologic aspects were correlated; contrast uptake was less in areas thought to be dense in collagen than it was in areas with more vascular regions of inflammatory change. CT assessed the extent and nature of the fibrotic plaque.