Objective— Adhesive interactions between cells and the extracellular matrix play an important role in inflammatory diseases like atherosclerosis. We investigated the role of the collagen-binding integrin α1β1 in atherosclerosis. Methods and Results— ApoE−/− mice were α1-deficient or received early or delayed anti-α1 antibody treatment. Deficiency in α1 integrin reduced the area of atherosclerotic plaques and altered plaque composition by reducing inflammation and increasing extracellular matrix. In advanced plaques, α1-deficient mice had a reduced macrophage and CD3+ cell content, collagen and smooth muscle cell content increased, lipid core sizes decreased, and cartilaginous metaplasia occurred. Anti-α1 antibody treatment reduced the macrophage content in initial plaques after early and delayed treatment, decreased the CD3+ cell content in advanced plaques after delayed treatment, and increased the collagen content in initial and advanced plaques after delayed treatment. Migration assays performed on α1-deficient macrophages on collagen I and IV substrata revealed that α1-deficient cells can migrate on collagen I, but not IV. Anti-α1 antibody treatment of ApoE−/− macrophages also inhibited migration of cells on collagen IV. Conclusions— Our results suggest that α1β1 integrin is involved in atherosclerosis by mediating the migration of leukocytes to lesions by adhesion to collagen IV. Blocking this integrin reduces atherosclerosis and induces a stable plaque phenotype.