The inhibitors of apoptosis (IAPs) family regulate apoptosis by preventing the action of the central execution phase, and function as mediators and regulators of the anti-apoptotic activity of the v-Rel and NF-kappaB transcription factor families. The targeting of IAPs may be a promising strategy, but it is not well elucidated in human hepatocellular carcinomas (HCCs). We have therefore investigated the effects of the down-regulation of IAPs (XIAP or survivin) on the TNF-related apoptosis-inducing ligand (TRAIL) and chemotherapeutic agents that induced apoptosis in human HCC cells. To inhibit the IAPs gene expression, we designed small interfering RNA (siRNA) against the X-chromosome-linked IAP (XIAP) or survivin and investigated their efficacy in the suppression of the XIAP or survivin expression in two HCC cells (SK-Hep1 and HLE), and their consequent antitumor potential. We found that the designed siRNAs against the XIAP and survivin downregulated the protein expression of respective genes by almost 50%. The suppression of IAPs resulted in a significant decrease in procaspase-3 levels, especially by suppression of the XIAP. The apoptosis cell count was small in cells transfected with control siRNA and siRNA against the XIAP or survivin, but after treatment with 10 ng/ml of TRAIL, the apoptosis cells increased 2-3 times by the suppression of IAPs as control. The cytotoxicity of doxorubicin and camptothecin was augmented by the suppression of the XIAP in SK-Hep1 cells, whereas the suppression of survivin did not affect cytotoxicity. In conclusion, downregulation of the XIAP or survivin enhances cell death by TRAIL and increases sensitivity against some chemotherapeutic agents in HCC cells. In particular, the XIAP may be a potential target to increase therapeutic sensitivity.