While the National Wilms' Tumor Study (NWTS) Group in the United States puts an emphasis on accurate staging and histology before any therapy is given for Wilms' tumor, the International Society of Pediatric Oncology (SIOP) in Europe focuses on preoperative therapy and safer surgery. Our current approach combines the benefits of both policies in the management of massive renal tumors in children. In seven consecutive patients we first obtained a percutaneous posterior needle biopsy to obtain adequate tissue for histology, and proceeded with preoperative chemotherapy with vincristine and dactinomycin until tumor shrinkage was sufficient. Tumor removals were feasible and uneventful. At the time of operation, two tumors were found to be totally or almost totally necrotic. In the others, which still included viable tumor, the histology corresponded well to the needle biopsy findings. One case with unfavorable histology and one with rhabdoid sarcoma would have been missed and given suboptimal therapy without the primary needle biopsy. As possible biopsy-related complications, subcapsular intratumoral bleeding was recognized in two patients. We conclude that percutaneous posterior needle biopsy is safe and yields definite, detailed histology in massive renal tumors in children. Preoperative chemotherapy facilitates surgery in these patients.