Wilms tumor weight was used to recruit patients in a recent National Wilms Tumor Study (NWTS) group trial. The authors hypothesized that a simple calculation of tumor volume based on a preoperative CT scan could predict tumor weight. The authors reviewed charts and CT images of patients with Wilms tumors who were treated at their institution between 1985 and 2002. Tumor volume was calculated as: V = 1/6pi x d (long axis) x d (short axis) x d (craniocaudal). Weight and calculated tumor volume were correlated using linear regression. Complete data of tumor weight and volume could be determined in 25 of the 49 patients. These were highly correlated (Spearman R = 0.97). Wilms tumor weight can be predicted based on a simple estimate of tumor volume on a preoperative CT scan. CT-estimated volume may replace weight as a prognostic factor and in guiding management.