Nephroblastomas (Wilms' tumors) are curable with survival rates above 80%. Nevertheless, some tumors fail to respond to therapy and those patients have a poor prognosis. Prognostic factors for nephroblastomas have still not been satisfactorily explored. In an effort to unravel molecular markers for non-responding nephroblastomas, we investigated by means of immunohistochemistty the expression of heat-shock protein 70 (HSP70) in formalin-fixed and paraffin-embedded tissue samples from 32 children afflicted with nephroblastoma. The results were validated using real-time RT-PCR. HSP70 expression was confined to blastemal and epithelial components, while the tumor stroma was negative. HSP70 expression was greater, if the tumors had been chemotherapeutically treated prior to operation, indicating that cytostatic drugs induce HSP70. Furthermore, high HSP70 expression was confined to tumors from children who survived, whereas tumors from dead patients were negative or weakly-positive for HSP70. Though the number of cases analyzed was small, they provide an indication that HSP70 expression may be of prognostic value.