The association between aniridia and Wilms' tumor (WT) is well known. The availability of data on a large number of WT patients entered on the first and second National Wilms' Tumor Studies provided the opportunity to review which examination had been most effective in detecting the presence of WT in patients being followed with aniridia. The records of 1287 patients were available and 13 or 1% had known aniridia. The details of the prediagnostic studies were available in 9/13 patients. Those nine patients had been examined regularly by IVP at intervals of 3-12 months prior to the discovery of the WT, but in only one was the unsuspected WT detected by these means. In the remaining eight patients, the tumor was discovered by palpation in six and suspected because of hematuria in two. It is concluded that repeated IVP's are not a satisfactory method to diagnose a WT at an early stage and newer radiodiagnostic studies, such as ultrasonography, will probably prove more effective.