The potential spaces of the mediastinum can be involved in a variety of disease processes and become evident by computed tomography study. The potential spaces may be distended with air, blood, inflammatory exudate, excessive fat, or tumor which separates and displaces the normal mediastinal structures. Seven cases with various pathologic processes are selected to demonstrate the computed tomography characteristics of such lesions. We emphasize the importance of computed tomography in evaluating and understanding various disease processes involving the potential spaces in an often widened mediastinum. © 1988.