Adult T-cell leukemia (ATL), an aggressive malignancy of CD4⁺ T cells associated with human T-cell leukemia virus type I (HTLV-I) infection, carries a very poor prognosis because of the resistance of leukemic cells to any conventional regimen, including chemotherapy. We examined the effect of ritonavir, an HIV protease inhibitor, on HTLV-I-infected T-cell lines and primary ATL cells and found that it induced apoptosis and inhibited transcriptional activation of NF-κB in these cells. Furthermore, ritonavir inhibited expression of Bcl-x_L, survivin, c-Myc, and cyclin D2, the targets of NF-κB. In nonobese diabetic/severe combined immunodeficient (NOD/SCID)/γc^null (NOG) mice, ritonavir very efficiently prevented tumor growth and leukemic infiltration in various organs of NOG mice at the same dose used for treatment of patients with AIDS. Our data indicate that ritonavir has potent anti-NF-κB and antitumor effects and might be clinically applicable for treatment of ATL. These results would provide a new concept and novel platform for new drug development of leukemia and solid cancer as well. (Blood. 2006;107:716-724)