Transforming growth factor beta (TGF-beta) has been shown to induce apoptotic cell death in normal and transformed hepatocytes. However, the exact mechanism through which TGF-beta induces cell death is still unknown. We examined a potential role of various death receptor/ligand systems in TGF-beta-induced apoptosis and identified the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) as a mediator of TGF-beta-induced apoptosis in hepatoma cells. TGF-beta-induced apoptosis is significantly impaired upon blockage of TRAIL. We show that TRAIL is upregulated in hepatoma cells upon treatment with TGF-beta, whereas TRAIL receptor levels remain unchanged. In conclusion, our results provide evidence that the TRAIL system is critically involved in TGF-beta-induced cell death in liver pathology.