The therapeutic strategy of the SIOP-9 study includes pre-operative chemotherapy for all patients with nephroblastoma diagnosed by imaging methods aged between 0.5 and 16 years. By pre-operative chemotherapy the rate of radical resectable tumors should be increased and thereby the intensity of postoperative therapy, in particular of radiotherapy, diminished. Patients with nephroblastoma stage I-III were in case of tumorresponse randomised in either a 4 weeks or 8 weeks arm of pre-operative treatment with ACT D and VCR. The question was, if a prolongation of pre-operative chemotherapy could increase the relative part of stage I. Between 1/1/89 and 6/30/91 from 49 oncologic centres of former western Germany 188 patients were registered in the SIOP-9/GPO. From the stage I-III patients between 0.5 and 16 years 80.1% were pretreated with cytostatic agents. Only 53.9% of the patients with tumorresponse were randomised. The relative frequency of intraoperative ruptures was with 3% lower in the group of pretreated patients than in the primary operated (15.3%). The stage distribution for all Wilms' tumor patients showed a prevalence of stage I with 43.3% (after pre-operative treatment 59%; after primary operation 28%). Abdominal radiotherapy was performed in 22.4%. The event-free survival rate of all nephroblastoma lay at 85% 3 years after diagnosis (stage I standard 96%; unfavorable histology all stages 45%). 7.3% of the patients developed a hepatopathy under treatment and 7.8% even a VOD according to the criteria of McDonald.(ABSTRACT TRUNCATED AT 250 WORDS)