Adult Wilms' tumor (AWT) is a very rare and aggressive malignancy, and little information is available on effective therapy in adults. Although mutations in WT1 have been found in 10% to 15% of childhood Wilms' tumor patients, to date WT1 mutations in AWT patients have not been described. The authors describe a 47-year-old man with relapsed AWT and a novel germline alteration in intron 1 of WT1: IVS1-6 C-->A. This alteration may reduce the splicing efficiency for exon 2 and possibly results in exon skipping. The effective salvage chemotherapy contained ifosfamide, carboplatin, and etoposide and was followed by a high-dose chemotherapy that contained melphalan, carboplatin, and etoposide. Both chemotherapy regimens showed moderate treatment-related toxicity. This report is the first that indicates that adult nephroblastoma patients also may carry WT1 germline mutations.