Computed tomographic (CT) scans and sonograms of 13 children with Wilms tumor were reviewed to determine the ability of each imaging test to characterize the tumor and determine its extent. The findings of this review were correlated with diagnoses based on surgical and pathologic evidence. Tumor necrosis and a pseudocapsule were detected more often using CT scans than sonograms. CT scanning also was more sensitive in assessing perinephric extension, lymph node involvement, and bilateral tumors. Overall, CT scans allowed better determination of the extent of a suspected tumor, enabling correct diagnosis in 77% of patients, while US study was correct in only 23%.