The ratio of pro-apoptotic (PAP) and anti-apoptotic (AAP) bcl-2 proteins is important in apoptosis regulation. We sought to determine if inhibition of the AAP bcl-xl by sodium butyrate (SB) would augment apoptotic cellular death in mesothelioma when combined with adenoviral pro-apoptotic gene therapy (PAGT) by simultaneously increasing PAP and decreasing AAP in these cells. Human mesothelioma cell lines were exposed to AdBax, AdBak, Adp53, and SB alone as well as all vectors combined with SB at varying doses and time points. Cell death was assessed, and apoptosis evaluated by morphology and FACS. Isobologram analysis evaluated additive or synergistic effect. Cellular death and apoptosis were augmented by PAGT/SB combinations compared to monotherapy. Following AdBax/SB and AdBak/SB, a decrease of the AAP bcl-xl was noted in combination with increases in PAP bax and bak. By isobologram analysis, additive or synergistic cell killing was noted with both combinations. SB treatment did not significantly augment cell killing or apoptosis in combination with Adp53. PAGT/SB was more effective than monotherapy in induction of apoptotic cell death. Synergy may be due to the ability of SB to decrease bcl-xl with marked increases in PAP engendered by PAGT. Combination therapy with agents that down-regulate AAP in addition to PAGT may prove useful clinically.