Recent reports indicate that the presence or absence of a periportal low-attenuation rim (''periportal collar'' sign) on CT scans is of variable reliability in predicting rejection after liver transplantation. To study this matter further, we reviewed 178 CT scans in 68 patients (74 allografts) after hepatic transplantation. One hundred twenty-one scans were obtained within 7 days of liver biopsy. The presence or absence of a central or peripheral periportal collar on these scans was correlated with the presence or absence of acute rejection found at biopsy. A central periportal collar was defined as an abnormally large amount of low-attenuation material surrounding the main portal vein, the right portal vein, or the left portal vein. A peripheral periportal collar was defined as a low-attenuation area surrounding a portal vein branch distal to the left or right portal veins. A weak, but statistically significant correlation between the presence of a peripheral periportal collar and acute allograft rejection was shown in patients in whom CT scans were obtained within 3 days of biopsy. However, the sensitivity, specificity, accuracy, and positive predictive value of the presence of a peripheral collar in this group were 60%, 67%, 64%, and 63%, respectively. The sensitivity, specificity, accuracy, and positive predictive value of the presence of a central collar in patients scanned within 3 days of biopsy were 74%, 35%, 54%, and 52%, respectively. Negative predictive values of peripheral and central collars were 65% and 59%, respectively. Our study shows that a periportal collar is seen more commonly in patients with recently transplanted allografts (mean interval after transplantation, 46 days for peripheral collars and 57 days for central collars) and in patients with ascites. The presence of a central or peripheral periportal collar on CT is not sufficiently sensitive, specific, or accurate to reliably diagnose or exclude acute allograft rejection.