Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a member of the TNF superfamily. TRAIL is promising for anticancer therapy because it induces apoptosis in cancer cells with little or no toxicity to normal cells; hence, TRAIL-receptor agonists are currently undergoing clinical trials for cancer treatment. However, many molecular signaling mechanisms in TRAIL signaling are not completely characterized. The functions of adaptor proteins, including TNF-receptor-associated death domain protein (TRADD) and receptor-interacting protein-1 (RIP1) in TRAIL signaling have been controversial. We demonstrate that while wild-type mouse embryonic fibroblasts (MEFs) are completely resistant to TRAIL-induced apoptosis, MEFs derived from Tradd(-/-) mice are hypersensitive to TRAIL (IC(50)~0.5 nM rmTRAIL, 24 h), an effect also seen in primary keratinocytes treated with TRAIL/CHX. Restoration of TRADD in Tradd(-/-) MEFs restores TRAIL resistance, indicating that TRADD plays a survival role in TRAIL signaling. We show that TRADD is recruited to the TRAIL-receptor complex, and RIP1 recruitment is mediated by TRADD. While early activation of the MAP kinase ERK is deficient in Tradd(-/-) cells, the main mechanism for enhanced TRAIL sensitivity is likely due to increased recruitment of FADD to the receptor complex, indicating that TRADD may limit FADD binding within the receptor complex and also mediate RIP1-dependent nonapoptotic signaling events, thus reducing caspase activation and subsequent apoptosis. These novel findings have potential implications for cancer therapy using TRAIL-receptor agonists.