The antiapoptotic proteins, Bcl-2 and Bcl-X_L, are expressed in most cases of acute myeloid leukemia (AML) and may contribute to drug resistance in AML. We tested the hypothesis that down-regulation of Bcl-2 alone by antisense oligodeoxynucleotides (Bcl-2-AS) induces apoptosis, even in the presence of other antiapoptotic genes. We tested Bcl-2-AS in myeloid leukemic HL-60 cells, in Bcl-2 and Bcl-X_L overexpressing HL-60-DOX cells, and in primary AML samples. Down-regulation of Bcl-2 by Bcl-2-AS reduced the viability of HL-60 cells and, less effectively, HL-60-DOX cells and increased ara-C cytotoxicity in both cell lines. Incubation of primary AML blasts with Bcl-2-AS decreased Bcl-2 expression in CD34⁺ blast cells after induction of apoptosis and enhancement of ara-C cytotoxicity in 11 of 19 primary AML samples. In 8 samples in which Bcl-2-AS did not induce apoptosis, baseline Bcl-2 levels were found to be strikingly high. The expression of other antiapoptotic proteins (Bcl-X_L, Bag-1, A1, and Mcl-1) did not prevent Bcl-2-AS–induced apoptosis. Bcl-2-AS also inhibited colony formation of AML progenitor cells. Low concentrations of Bcl-2-AS induced significant increases in S-phase cells (P = .04). Results establish Bcl-2 as a critical target for AS strategies in AML in which the baseline levels predict response to Bcl-2-AS. Bcl-2 exerts both antiapoptotic and antiproliferative functions in AML. Because early normal hematopoietic stem cells do not express Bcl-2, Bcl-2-AS therapy should be highly selective for AML cells.