Purpose: Evaluate the response of human pancreatic cancer cell lines and orthotopic tumors to TRA-8, an agonistic antibody to death receptor 5, in combination with irinotecan (CPT-11). Experimental Design: MIA PaCa-2 and S2VP10 cells were treated with TRA-8 and/or CPT11. Cell viability was determined by ATP assay. JC-1 mitochondrial depolarization and Annexin V assays confirmed cell death by apoptosis. Immunoblotting was used to evaluate protein changes. MIA PaCa-2 cells were injected into the pancreas of severe combined immunodeficient mice. Mice underwent abdominal ultrasound to quantitate tumor size before and after treatment with twice weekly injections of 200 μg TRA-8 and/or 25 mg/kg CPT-11 for one or two treatment cycles, each lasting 2 weeks. Results: MIA PaCa-2 cells were more sensitive to TRA-8 and showed additive cytotoxicity, whereas S2VP10 cells showed synergistic cytotoxicity when treated with TRA-8 and CPT-11. Cell death occurred via apoptosis with increased cleavage of caspase-3, caspase-8, and caspase-9 and proapoptotic proteins Bid and poly(ADP)ribose polymerase after combination treatment compared with either agent alone. XIAP and Bcl-XL inhibitors of apoptosis were down-regulated. After a single cycle of in vivo combination therapy, tumor sizes had diminished significantly (P < 0.001) at 8 days posttreatment compared with no treatment, CPT-11, and TRA-8; and there was a 50-day increase in survival with combination treatment over untreated controls (P = 0.0002), 30 days over TRA-8, and a 36-day increase over CPT-11 monotherapy (P = 0.0003). With two cycles of TRA-8/CPT-11 treatment, mean survival time increased significantly (P < 0.001) to 169 days versus untreated controls, TRA-8 or CPT-11 (76, 121, or 108 days, respectively). Conclusions: Combination TRA-8 and CPT-11 therapy produced enhanced cytotoxicity and survival in the MIA PaCa-2 orthotopic model of pancreatic cancer. © 2007 American Association for Cancer Research.