STI571/Gleevec/imatinib, a rationally-designed agent that occupies the ATP-binding site of BCR-ABL and stabilizes the protein in its closed, inactive conformation, has been a remarkable success for the treatment of chronic myeloid leukemia (CML). However, a significant proportion of patients chronically treated with STI571 develop resistance because of the acquisition of mutations in the kinase domain of BCR-ABL. Furthermore, the effects of STI571 on CML patients in accelerated phase or blastic crisis are unsatisfactory since many patients relapse after transient remission. Hence, additional drugs or STI571-based combination regimens are desired to circumvent resistance and to improve response rates. Here we reported that PS-341, a proteasome inhibitor which offers great promise to patients with multiple myeloma (MM), significantly enhanced the antileukemia activity of STI571 in vitro and in vivo. We found a synergy exists between low concentrations of PS-341 (5–10 nM) and STI571 (0.1–0.2 μM) in inhibition of cell growth and induction of apoptosis in K562 cell line and CD34⁺ leukemic cells isolated from CML patients. In K562 cells, combined use of PS-341 and STI571 accelerated activation of caspase-3, 9, and facilitated cleavage of poly-(ADP-ribose) polymerase (PARP) as compared to those in cells treated with PS-341 or STI571 alone. Moreover, PS-341/STI571 combination resulted in potentiated degradation of BCR-ABL and downregulation of phosphorylated BCR-ABL as compared to those in mono treatment. In nude mice inoculated subcutaneously with K562 cells, treatment with PS-341 (injected intraperitoneally, ip) alone (at doses of 0.05, 0.5, 1 mg/kg/d, twice a week for 4 weeks, respectively) decreased tumor growth in a dose-dependent manner. STI571 (ip) at 10 mg/kg/d also inhibited tumor growth. Intriguingly, combinatory administration of low dose PS-341 (0.05 mg/kg/d, twice a week for 4 weeks) and STI571 (10 mg/kg/d) yielded a much more profound inhibition of tumor growth and even clearance of leukemic cells in mice compared to either monotherapy. Taken together, these results demonstrate synergic effects of PS-341 and STI571, and provide the rationale to evaluate PS-341/STI571 combination in treating CML aiming to further improve clinical outcome of patients.