Sodium butyrate as a histone deacetylase inhibitor is known to exhibit anti-cancer effects via the differentiation and apoptosis of various carcinoma cells. However, the mechanism by which sodium butyrate induces apoptosis and the involvement of telomerase activity during apoptosis is not completely understood. To investigate the underlying pathways, sodium butyrate's potential to induce apoptosis in human leukemic U937 cells and its effects on telomerase activity were investigated. Exposure of U937 cells to sodium butyrate resulted in growth inhibition and induction of apoptosis in a dose-dependent manner as measured by hemocytometer counts, fluorescence microscopy, agarose gel electrophoresis and flow cytometry analysis. The increase in apoptosis was associated with the up-regulation in pro-apoptotic Bax expression, and down-regulation of anti-apoptotic Bcl-2 and Bcl-XL. Sodium butyrate treatment also inhibited the levels of cIAP family members and induced the activation of caspase-3. Furthermore, sodium butyrate markedly inhibited the activity of telomerase and the expression of human telomerase reverse transcriptase (hTERT), a main determinant of the telomerase enzymatic activity, was progressively down-regulated by sodium butyrate. Taken together, it is suggested that sodium butyrate can be a promising chemopreventive agent for leukemic cells and changes in Bcl-2 family expressions, as well as telomerase activity may, play critical roles in sodium butyrate-induced apoptosis in U937 cells.