Abstract Accumulation of unfolded or misfolded proteins within the endoplasmic reticulum triggers the unfolded protein response (UPR). Evidence from several studies suggests that the UPR is activated in various tumors and might play a crucial role in tumor growth. However, the role of the UPR in leukemia remains unclear. Therefore, to define the role of the UPR in leukemogenesis, we used p210 Bcr-Abl-expressing 32D myeloid cell lines (p210 32D). The mRNA expression of UPR-related genes, namely, a spliced form of X-box DNA-binding protein (XBP1) and glucose regulated protein 78 (GRP78), was increased in p210 32D. Luciferase assay indicated that p210 Bcr-Abl induced high levels of the transcriptional activity of the UPR element. Moreover, levels of the phosphorylated eIF2 alpha protein increased in p210 32D. Inhibition of the UPR using IRE1 and ATF6 dominant-negative mutants diminished the ability of Bcr-Abl to protect the cells from etoposide- and imatinib-induced apoptosis, but had no effect on the proliferation of p210 Bcr-Abl-transformed cells. We also evaluated the expression of UPR-related genes in primary leukemic cells from Ph chromosome-positive cells by real-time RT-PCR; it was found that these cells showed higher expression levels than the control cells. Taken together, these results for the first time suggested that UPR is a downstream target of the Bcr-Abl oncoprotein, and it plays the anti-apoptotic role of Bcr-Abl in Ph chromosome-positive leukemia cells.