Sclerosing retroperitonitis and sclerosing mediastinitis are fibrotic lesions of soft tissue that simulate clinically certain types of malignant neoplasms. The medical history of a patient having either of these diseases is that of a patient having a malignant disorder. Sclerosing retroperitonitis occludes renal outflow, and sclerosing mediastinitis obstructs venous inflow in the thorax. Sclerosing retroperitonitis"' ' ' "'-'"' 33,36.37,39 j i a s |-,een documented 45 times in the American literature; sclerosing mediastinitis' »• "• ' "• 1 ' 2 ' 4 ' 6 has been described less frequently, appearing in foreign as well as in American reports. The importance of distinguishing sclerosing retroperitonitis and sclerosing mediastinitis from malignant disease is obvious. Moreover, there is a distinct impression expressed by our clinical colleagues that these 2 sclerosing disorders are increasing in incidence. I t is the authors' purpose in this paper to discuss the various features of the 2 disorders as observed in 6 patients, and to catalogue examples of the disorders from the literature.