Background: Genome-wide association studies have identified the chromosome 9p21 locus as one of the most important genetic risk factors for cardiovascular disease. However, the mechanism by which this locus promotes disease remains unclear due to difficulty identifying the causal genes and lack of a suitable animal model. Methods and Results: A total of 180 coronary artery autopsy specimens were analyzed histopathologically. The genotype of 9p21 (rs1333049) was not associated with any coronary risk factors nor the vulnerable plaque phenotype, but carriers of 9p21 risk allele did demonstrate more lesional calcification. To extend these studies into an animal model, mice with a targeted deletion of the orthologous 70-kb non-coding interval on chromosome 4 (chr4^D70kb/D70kb) were bred onto ApoE^-/- and fed with high fat diet. Targeted deletion of the 9p21 risk interval increased susceptibility to atherosclerotic plaque progression, but did not affect plaque rupture in the tandem stenosis model. Coronary risk factors such as body weight, blood pressure, lipid, and glucose levels did not differ between genotypes. Von Kossa staining revealed that chr4^D70kb/D70kb, ApoE^-/- developed more calcification in the plaque compared with chr4^+/+, ApoE^-/- mice, and that this this change was accompanied by increased aortic mRNA expression of Runx2, a key osteogenic transcription factor. Primarily cultured smooth muscle cells from chr4^D70kb/D70kb were hyperproliferative, and showed a calcification-prone phenotype after exposure to high-phosphate medium. Treatment with Palbociclib, a selective inhibitor of cyclin-dependent kinase 4/6, reduced mRNA expression of osteogenic genes in smooth muscle cells. Conclusion: Results from human and mouse studies indicate that the 9p21 non coding risk interval is associated with larger atherosclerotic plaque burden, but not with plaque rupture. 9p21 may promote lesion expansion by inducing the proliferation of de-differentiated and osteogenic smooth muscle cells.