Unilateral "vanishing psoas": an anatomic variant.

We report five patients found to have obvious asymmetry of the psoas muscles on abdominal computed tomography (CT) examination. In all five patients, the upper portion of the psoas muscle was normal, but the lower portion was significantly reduced in size. In no patient was there neurological or other disease that might explain this finding. No patient exhibited signs or symptoms secondary to psoas asymmetry. Asymmetry of the psoas musculature is apparently a benign anatomical variant that may be recognized on CT examination and must be differentiated from pathological conditions.