Bortezomib (Velcade, formerly known as PS-341) is a boronic acid dipeptide derivative, which is a selective and potent inhibitor of the proteasome. We examined the antitumor activity of combination therapy with bortezomib + docetaxel in two human pancreatic cancer cell lines (MiaPaCa-2 and L3.6pl) selected for their divergent responses to bortezomib alone. Bortezomib blocked docetaxel-induced apoptosis in the MiaPaCa-2 cells and failed to enhance docetaxel-induced apoptosis in L3.6pl cells in vitro but did interact positively with docetaxel to inhibit clonogenic survival. These effects were associated with decreased accumulation of cells in M phase, stabilization of the cyclin-dependent kinase inhibitors, p21 and p27, and inhibition of cdk2 and cdc2 activities. In orthotopic xenografts, combination therapy produced significant reductions in tumor weight and volume in both models associated with accumulation of p21, inhibition of proliferation, and increased apoptosis. Combination therapy also reduced tumor microvessel densities, effects that were associated with reductions in tumor cell production of vascular endothelial growth factor and increased levels of apoptosis in tumor-associated endothelial cells. Together, our results suggest that bortezomib enhances the antitumoral activity of taxanes by enforcing cell growth arrest and inhibiting angiogenesis.