The potential for resection with renal salvage, and the ability of computed tomographic (CT) criteria to predict this potential, were evaluated retrospectively in 43 children with nonmetastatic unilateral Wilms' tumor (stage I, II, or III at diagnosis). CT criteria for renal salvage were (a) tumor involving only one pole and occupying less than one third of the kidney; (b) functioning kidney; (c) no invasion of collecting system or renal vein; and (d) clear margins between tumor, kidney, and surrounding structures. Preoperative CT scans met these criteria in only two of the 43 cases. Surgical and/or pathologic reports confirmed CT-based findings in all cases. Most of the tumors were large (median vertical and transverse diameters both = 10 cm), and only three patients were estimated to have more than 50% functioning renal tissue. A prospective trial would be necessary to evaluate the long-term benefits and possible complications of renal salvage procedures in patients with Wilms' tumor. Given the small proportion of patients likely to be eligible for such procedures, and the currently excellent cure rates in low-stage disease, such as trial would be difficult to implement.