In patients surgically proven to have a Wilms' tumor, preoperative diagnostic accuracy was excellent in this hospital, and treatment was promptly undertaken in 41 cases seen over a 17 year period. However, an erroneous diagnosis of Wilms' tumor was made in 8 patients. These 8 renal lesions were benign, representing various congenital, infectious, obstructive, and ischemic processes. Four of the 8 patients received unnecessary preoperative irradiation. A retrospective study of these cases suggests that many diagnostic errors of this particular type may be avoidable, or at least the preoperative diagnosis may be sufficiently uncertain to discourage preoperative irradiation.