Human T-cell leukemia virus type I (HTLV-I) is the causative agent of an aggressive form of leukemia designated adult T-cell leukemia (ATL). We have previously demonstrated that all T-cell lines infected with HTLV-I and primary leukemic cells from ATL patients display constitutively high activity of transcription factor NF-kappaB. In this study we showed that Bay 11-7082, an inhibitor of NF-kappaB, induced apoptosis of HTLV-I-infected T-cell lines but only negligible apoptosis of HTLV-I-negative T cells. Bay 11-7082 rapidly and efficiently reduced the DNA binding of NF-kappaB in HTLV-I-infected T-cell lines and down-regulated the expression of the antiapoptotic gene, Bcl-x(L), regulated by NF-kappaB, whereas it had little effect on the DNA binding of another transcription factor, AP-1. Although the viral protein Tax is an activator of NF-kappaB, Bay 11-7082-induced apoptosis of HTLV-I-infected cells was not associated with reduced expression of Tax. Furthermore, Bay 11-7082- induced apoptosis of primary ATL cells was more prominent than that of normal peripheral blood mononuclear cells, and apoptosis of these cells was also associated with down-regulation of NF-kappaB activity. Our results indicate that NF-kappaB plays a crucial role in the pathogenesis and survival of HTLV-I-infected leukemic cells and that it is a suitable target for the prevention and treatment of ATL.