Human T lymphotropic virus type I (HTLV-I) is the etiological agent of adult T-cell lymphocytic leukemia (ATLL), whereas HTLV-II has not been associated with hematopoietic malignancies. The control of apoptotic pathways has emerged as a critical step in the development of many cancer types. As a result, the underlying mechanism of long-term survival of HTLV-I and HTLV-II was studied in infected T cells in vitro and in ex vivo ATLL samples. Results indicate that HTLV-I- and HTLV-II-infected T cells in vitro express high levels of the antiapoptotic protein Bcl compared with other human leukemic T cell lines or uninfected peripheral blood mononuclear cells. The levels of proapoptotic proteins Bax, BAD, and Bak were not significantly altered. HTLV-I and HTLV-II viral transactivators, Tax1 and Tax2, are known to increase expression of cellular genes. These proteins were tested for increased transcription from the human Bcl2 and Bcl-X(L) promoters. Whereas no effect was observed on the Bcl2 promoter, both Tax1 and Tax2 increased transcription of the Bcl-X(L) promoter in T cells, although Tax1 appeared to be more efficient than Tax2. The biological significance of these observations was validated by the finding of an increased expression of Bcl-X(L) in ex vivo ATLL cells, especially from patients unresponsive to various chemotherapy regimens. Altogether, these data suggest that overexpression of Bcl-X(L )in vivo( )may be in part responsible for the resistance of ATLL cells to chemotherapy. In addition, inefficient activation of the Bcl-X(L) promoter by Tax2 may result in a shorter survival time of HTLV-II-infected cells in vivo and a diminished risk of leukemia development.