The previous International Society of Paediatric Oncology (SIOP) trials and studies recognized three prognostic groups of renal tumors of childhood: low risk, intermediate risk, and high risk tumors, which were further defined in the SIOP (Stockholm) Working Classification of Renal Tumors of Childhood (1994). The results of the latest SIOP Trials and Studies showed that certain histological features which remain after preoperative chemotherapy, such as blastema, are of prognostic significance while others are not. Therefore, in the next SIOP Trials and Study a revised classification of renal tumors will be followed. It still recognizes the three tumor risk groups with different types in each of them, but for treatment purposes, only three major types of nephroblastoma need to be recognized: completely necrotic (low risk tumor), blastemal (high risk tumor), and others (intermediate risk tumors). Patients will be treated according to tumor histology and stage. Trials which include preoperative chemotherapy have shown that the presence of necrotic tumor or chemotherapy induced changes in the renal sinus or perirenal fat can be ignored for distinguishing between stage I and II, but if present at resection margins or lymph nodes, it should be regarded as stage III. Prognostic significance of all histological component of Wilms tumors will be studied prospectively in the new trial. Med Pediatr Oncol 2002;38:79–82. © 2002 Wiley-Liss, Inc.