The effects of sodium butyrate (SB), a histone deacetylase inhibitor, in combination with cisplatin (CDDP), for inhibition of cell growth and induction of apoptosis were investigated in bladder cancer cell lines in vitro. Bladder cancer cell lines T24, 253J, and UMUC3 were treated with different concentrations of CDDP or SB. Cell proliferation was studied by XTT assay. Cell-cycle analysis and induction of apoptosis were analyzed by laser scanning cytometry (LSC). Western blot analysis was used to determine expression of p21, p27, TRADD, FADD, caspase-2, and caspase-7. We observed that SB in combination with CDDP induced significant inhibition of cell growth in a dose-dependent manner through G₁ arrest and apoptosis, as determined by LSC. When bladder cancer cell lines were treated with SB plus CDDP, Western blotting showed increased expression of p21 but not p27 in T24 cells, whereas both p21 and p27 increased in 253J and UMUC3 cells. All cell lines exhibited a moderate increase in TRADD and decrease in procaspase-2 but no significant change in FADD and procaspase-7. The results showed the synergistic anticancer effect of SB in combination with CDDP, their potential for treatment of bladder cancer, and their mechanism of action in terms of cell signal transduction and induction of apoptosis.