A low-attenuation region around the peripheral portal tracts (periportal collar) was noted on 13 of 43 computed tomographic studies obtained in 17 patients who had undergone 20 orthotopic liver transplantations. This region was then correlated with acute liver transplant rejection. The periportal collar as a sign of rejection has a sensitivity of 1.0, a specificity of 0.86, a negative predictive value of 1.0, and a positive predictive value of 0.62. This sign corresponds histopathologically with the lymphocytic portal infiltration that occurs during acute liver transplant rejection. Though viral hepatitis and nonspecific portal triad edema can also result in a periportal collar, acute liver rejection is strongly suggested when this sign is seen after liver transplantation. Early diagnosis and confirmation of rejection permit faster and more appropriate clinical intervention.