Fifty-eight unilateral anaplastic Wilms' tumors entered on the Third National Wilms' Tumor Study (NWTS-3) were reviewed. The favorable outcome of stage I anaplastic tumors was confirmed. In stage II-IV tumors, the extent and severity of anaplasia were not of prognostic importance. However, two histologic features were correlated with extremely poor outcome; anaplasia in extrarenal tumor sites (P = .016) and predominantly blastemal tumor pattern (P = .020). The importance of these features in predicting relapse and death for children with anaplastic Wilms' tumor was supported by the study of 47 additional anaplastic Wilms' tumor cases from NWTS-2. These results emphasize the importance of examining sites of extrarenal spread in anaplastic Wilms' tumors. Cytologic criteria for anaplasia and clues to its recognition are reviewed, and the potential clinical and biological implications of our findings are discussed.