Purpose: To retrospectively determine the diagnostic performance of computed tomography (CT) in identifying the presence or absence of preoperative Wilms tumor rupture. Materials and Methods: The cohort was derived from the AREN03B2 study of the Children's Oncology Group. The study was approved by the institutional review board and was compliant with HIPAA. Written informed consent was obtained before enrollment. The diagnosis of Wilms tumor rupture was established by central review of notes from surgery and/or pathologic examination. Seventy Wilms tumor cases with rupture were matched to 70 Wilms tumor controls without rupture according to age and tumor weight (within 6 months and 50 g, respectively). CT scans were independently reviewed by two radiologists, and the following CT findings were assessed: poorly circumscribed mass, perinephric fat stranding, peritumoral fat planes obscured, retroperitoneal fluid (subcapsular vs extracapsular), ascites beyond the cul-de-sac, peritoneal implants, ipsilateral pleural effusion, and intratumoral hemorrhage. All fluids were classified as hemorrhagic or nonhemorrhagic by using a cutoff of 30 HU. The relationship between CT findings and rupture was assessed with logistic regression models. Results: The sensitivity and specificity for detecting Wilms tumor rupture were 54% (36 of 67 cases) and 88% (61 of 69 cases), respectively, for reviewer 1 and 70% (47 of 67 cases) and 88% (61 of 69 cases), respectively, for reviewer 2. Interobserver agreement was substantial (k = 0.76). All imaging signs tested, except peritoneal implants, intratumoral hemorrhage, and subcapsular fluid, showed a significant association with rupture (P = .02). The attenuation of ascitic fluid did not have a significant correlation with rupture (P = .9990). Ascites beyond the cul-de-sac was the single best indicator of rupture for both reviewers, followed by perinephric fat stranding and retroperitoneal fluid for reviewers 1 and 2, respectively (P < .01). Conclusion: CT has moderate specificity but relatively low sensitivity in the detection of preoperative Wilms tumor rupture. Ascites beyond the cul-de-sac, irrespective of attenuation, is most predictive of rupture. (c) RSNA, 2012 Supplemental material: http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.12120670/-/DC1.