Smooth muscle cell (SMC) accumulation is a key event in the development of atherosclerosis, including vein bypass graft arteriosclerosis. Because members of the protein kinase C (PKC) family signal cells to undergo proliferation, differentiation, or apoptosis, we generated PKCδ knockout mice and performed vein bypass grafts on these animals. PKCδ–/– mice developed normally and were fertile. Vein segments from PKCδ–/– mice isografted to carotid arteries of recipient mice of either genotype led to a more severe arteriosclerosis than was seen with PKCδ+/+ vein grafts. Arteriosclerotic lesions in PKCδ–/– mice showed a significantly higher number of SMCs than were found in wild-type animals; this was correlated with decreased SMC death in lesions of PKCδ–/– mice. SMCs derived from PKCδ–/– aortae were resistant to cell death induced by any of several stimuli, but they were similar to wild-type SMCs with respect to mitogen-stimulated cell proliferation in vitro. Furthermore, pro-apoptotic treatments led to diminished caspase-3 activation, poly(ADP-ribose) polymerase cleavage, and cytochrome c release in PKCδ–/– relative to wild-type SMCs, suggesting that their apoptotic resistance involves the loss of free radical generation and mitochondrial dysfunction in response to stress stimuli. Our data indicate that PKCδ maintains SMC homeostasis and that its function in the vessel wall per se is crucial in the development of vein graft arteriosclerosis.