The sonographic records of 27 children who underwent a Cohen ureteroneocystostomy were reviewed. In 13 children (48%) sonograms demonstrated an echogenic, nonacoustically shadowing structure at or just above the trigone that was fixed in position within the bladder wall and covered by intact mucosa. This constellation of sonographic findings, which represent the submucosal segment of the reimplanted ureter, produces a "tunnel sign." Awareness of this appearance will enable the sonographer to better evaluate children who have undergone surgical procedures on the bladder.