Although the p53 anti-oncogene is an important target for gene therapy of cancer, some cancers are resistant to p53 gene transfer. For this reason, it is important to find effective drugs to enhance cytotoxic effects of p53 gene transfer. Recent reports demonstrated that some histone deacetylase inhibitors in combination with p53 gene therapy induced apoptosis in certain cancer cells more efficiently than p53 gene therapy alone. We investigated whether histone deacetylase inhibitor trichostatin A (TSA), in combination with p53 gene transfer could synergistically induce apoptosis in the breast cancer cell line MDA-MB-231. Whereas the adenovirus-expressing p53 (Ad-p53) by itself at up to 100 multiplicity of infection (MOI) induced apoptosis at a low level, Ad-p53 in combination with TSA synergistically induced apoptosis at a higher level in MDA-MB-231 cells than TSA or Ad-p53 alone. However, the combination of Ad-p53 and TSA did not enhance the expressions of p53 or p53-induced genes that are involved in apoptosis, and synergistically reduced the mitochondrial membrane potential and enhanced caspase-3 activity. These results suggest that TSA have synergistic effects on the induction of apoptosis in MDA-MB-231 cells when combined with p53 gene transfer.