Rho kinases (ROCKs) are serine-threonine protein kinases that regulate the actin cytoskeleton. Recent studies suggest that ROCKs also play an important role in cardiovascular disease. However, the isoform-and tissue-specific role of ROCKs in mediating this process is unknown. Using homologous recombination, we generated mutant mice harboring alleles with homozygous deletion of ROCK1 (ROCK1−/−). Most ROCK1−/− mice die perinatally. However, a few ROCK1−/− mice survive to adulthood, are phenotypically normal, and have no apparent compensatory changes in ROCK2. Using these ROCK1−/− mice, we show that ROCK1 in bone marrow-derived macrophages is critical to the development of atherosclerosis, in part, by mediating foam cell formation and macrophage chemotaxis. Lipid accumulation and atherosclerotic lesions were reduced in atherosclerosis-prone LDLR−/− mice, whose bone marrows have been replaced with bone marrows derived from ROCK1−/− mice. Bone marrow-derived ROCK1-deficient macrophages exhibited impaired chemotaxis to monocyte chemotactic protein-1 and showed reduced ability to take up lipids and to develop into foam cells when exposed to modified low-density lipoprotein. These findings indicate that ROCK1 in bone marrow-derived cells is a critical mediator of atherogenesis and suggest that macrophage ROCK1 may be an important therapeutic target for vascular inflammation and atherosclerosis.—Wang, H.-W., Liu, P.-Y., Oyama, N., Rikitake, Y., Kitamoto, S., Gitlin, J., Liao, JK, Boisvert, WA Deficiency of ROCK1 in bone marrow-derived cells protects against atherosclerosis in LDLR−/− mice.