Palliative chemotherapy with gemcitabine, a common mode of treatment of pancreatic cancer, has little influence on patients' survival. We investigated the impact of anti‐apoptotic Bcl‐xL protein and its antagonist Bax on gemcitabine‐induced apoptosis in human pancreatic carcinoma cells in vitro and in vivo. The level of Bcl‐xL and Bax expression was determined in 3 established pancreatic cancer cell lines that differ in their sensitivity to gemcitabine‐mediated apoptosis. Bcl‐xL and Bax genes were transduced into Colo357 cells by retroviral infection. In addition, cells were transfected with c‐FLIP to assess involvement of CD95 and caspase‐8. The impact of Bax/Bcl‐xL expression on gemcitabine‐sensitivity in vivo was evaluated in orthotopic Colo357 tumors in SCID mice. The apoptotic index revealed a strong inverse correlation between Bcl‐xL expression and gemcitabine‐induced apoptosis in the pancreatic carcinoma cell lines tested. Caspase‐8 and Bid were cleaved in Colo357 cells exposed to gemcitabine, and there was no correlation with either Bcl‐xL or with Bax expression. In contrast, the lack of mitochondrial transmembrane potential transition, release of cytochrome‐c and absence of caspase‐9‐ and PARP‐cleavage showed a strong correlation with Bcl‐xL expression. Expression of c‐FLIP significantly increased the resistance towards gemcitabine. Orthotopically growing Colo357‐bcl‐xl tumors in SCID mice were refractory to gemcitabine treatment, and in contrast to the in vitro data, Colo357‐bax tumors exhibited a 12‐fold greater tumor regression than Colo357‐wild‐type tumors in the control group. Gemcitabine‐induced apoptosis involves the mitochondria‐mediated signaling pathway. A functional restoration of this pathway appears to be essential to overcome the resistance mechanisms of pancreatic tumor cells and to improve the response to therapy as demonstrated by Bax overexpression in a clinically relevant tumor model. © 2003 Wiley‐Liss, Inc.