Fifty patients with disease involving the psoas compartment were analyzed. There were 27 patients with inflammation, 17 with tumor, and 6 with hemorrhage. In all but 3 cases the disease did not originate in the psoas but spread there from neighboring structures. Disease processes followed the fascial planes in patients with abscesses and hemorrhage but were less predictable with tumors. All three types of lesions were similar radiographically and were often indistinguishable. Asymmetry of the muscle mass, regions of decreased density, and opacification of fat in the peri- and pararenal compartments were the most frequent signs of disease. Computed tomography is recommended as the diagnostic procedure of choice.