In SIOP trials, Wilms' tumors were labeled as stage II by the presence of nonviable and/or viable tumor in the renal sinus and/or perirenal fat. The aim of this study was to determine if this approach was justified. Stage II Wilms' tumors were reviewed to establish whether staging was due to viable or nonviable tumor, and this was related to clinical outcome. One hundred sixty-nine patients were included: 40 had stage II due to the presence of nonviable tumor and 129 due to viable tumor. Postoperatively, 29 patients were undertreated: 7 with nonviable and 22 with viable stage II tumors. No undertreated patient with nonviable stage II relapsed or died (event-free survival [EFS] and overall survival [OS] 100%), whereas 3 of 22 with viable stage II relapsed, and 2 of them died (EFS 86%, OS 91%). Of 140 correctly treated patients, only 1 of 33 nonviable stage II patients relapsed and died (EFS and OS 97%); 8 of 107 patients with viable stage II relapsed (EFS 92%), and 3 of them died (OS 97%). The presence of nonviable tumor in the renal sinus and/or perirenal fat does not predict an adverse outcome in Wilms' tumors, and alone it does not warrant designation to stage II.