Response rates in relapsed Wilms' tumor. A need for new effective agents. Three hundred eighty-one children with Wilms' tumor were treated in the United Kingdom Children's Cancer Study Group WT1 Study (1982 to 1986). Seventy-one patients had relapses during or after treatment with surgery and chemotherapy, and radiation therapy, depending on stage and histologic characteristics. Forty-nine patients were evaluable for disease response to second-line chemotherapy alone. Evaluation of response to chemotherapy was impossible in the remaining patients because either surgery or radiation therapy was used at the time of relapse. With second-line combination chemotherapy (which included ifosfamide, etoposide/VM26, cisplatin/carboplatin, bleomycin, melphalan, and Thiotepa [Lederle Laboratories, Pearl River, NY]), there were five complete responses and 12 partial responses. In patients with favorable histologic findings, six of nine with Stage I, five of ten with Stage II, none of 11 with Stage III, three of 16 with Stage IV, and one of five with Stage V disease survived. Two survivors were treated with chemotherapy alone; the others received combined treatment with chemotherapy, radiation therapy, and/or surgery. For those with unfavorable histologic findings of any stage, only two of 20 survived. The authors conclude that, even for patients with localized disease with favorable histologic findings, the "salvage" rate is little more than 50%, and for all other stages and histologic findings the likelihood of cure after relapse is remote. There is clearly a need for additional effective chemotherapeutic agents for these patients. 