Programmed cell death (apoptosis) seems to be the principal mechanism whereby anti-oncogenic therapies such as chemotherapy and radiation effect their responses. Resistance to apoptosis, therefore, is probably a principal mechanism whereby tumors are able to overcome these cancer therapies. The transcription factor NF-κB is activated by chemotherapy and by irradiation in some cancer cell lines. Furthermore, inhibition of NF-κB in vitro leads to enhanced apoptosis in response to a variety of different stimuli. We show here that inhibition of NF-κB through the adenoviral delivery of a modified form of IκBα, the inhibitor of NF-κB, sensitizes chemoresistant tumors to the apoptotic potential of TNFκ and of the chemotherapeutic compound CPT-11, resulting in tumor regression. These results demonstrate that the activation of NF-κB in response to chemotherapy is a principal mechanism of inducible tumor chemoresistance, and establish the inhibition of NF-κB as a new approach to adjuvant therapy in cancer treatment.