Heat shock protein 70 (HSP70), an antiapoptotic chaperon protein, is highly expressed in human breast tumors and renders them resistant to such therapy as hyperthermia. In the present study, we inhibited the expression of HSP70 by blocking the heat shock transcription factor 1 (HSF1) function with its dominant-negative mutant (mHSF1) in Bcap37 cells, a thermo-tolerant breast cancer cell line. Here we report that retrovirus-mediated transfer of mHSF1 led to massive cell death of Bcap37 after hyperthermia. mHSF1 sensitized Bcap37 cells to hyperthermia by promoting apoptosis induced by heat shock. We also examined the efficacy of mHSF1 gene therapy in the nude mouse. mHSF1 transfection led to diminution of tumor growth with hyperthermia therapy. Thus, disrupting HSF1 in combination with hyperthermia may open new possibilities for treatment of cancers that have acquired resistance to heat treatment. (C) 2002 Elsevier Scienre (USA).