In general, the osoas sign has been known to be a reliabler index of presence of a retroperitioneal pathology. However, obliterated psoas margin may be caused by various other conditons in so far as the amount of fat around the psoas muscle is not enough to be visualized. On the other hand, retroperitoneal pathology does not always obliterates the psoas margin. Authors analized obliterated psoas margins in 72 patients by comparing simple radiographs and computed tomography, and attempted to explain the mechanizm of obliterated psoas margin, on simple radiograph. RESULTS : 1. The psoas margin is obliterated by the retroperitioneal pathology and various other conditions such as kidney-psoas contact, scanty extraperitoneal fat, scoliosis, bowel interposition and angled psoas muscle. 2,. The psoas margin is preserved as far as the perinephric fat is intact and X-ray beam strikes the lateral margin of thepsoas muscle tangentially. 3. The psoas sign is considered not to be a reliabl index of a retroperitoneal pathology.