In general, the psoas sign has been known to be a reliabler index of presence of a retroperitoneal pathology.However, obliterated psoas margin may be caused by various other conditions in so far as the amount of fat aroudthe psoas muscle is not enough to be visualized. On the other hand, retroperitoneal pathology does not alwaysobliterated the psoas margin. Authors analized obliterated psoas margins in 72 patients by comparing simpleradiographs and CT, and attempted to explain the mechanism of obliterated psoas margin, on simple radiograph. Theresults are as follows: 1. The psoas margin is obliteratd by the retroperitoneal pathology and various otherconditions such as kidney-psoas contact, scantly extraperitoneal fat, scoliosis, bowel interposition and angledpsoas muscle. 2. The psoas margin is preserved as far as the perinephric fat is intact and X-ray beam strikes thelateral margin of the psoas muscle tangentially. 3. The psoas sign is considered not to be a reliable index of aretroperitoneal pathology.