OBJECTIVE—Platelets are important for the development and progression of atherosclerotic lesions. However, relatively little is known about the contribution of platelet signaling to this pathological process. Our recent work identified 2 independent, yet synergistic, signaling pathways that lead to the activation of the small GTPase Rap1; one mediated by the guanine nucleotide exchange factor, CalDAG-GEFI (CDGI), the other by P2Y12, a platelet receptor for adenosine diphosphate and the target of antiplatelet drugs. In this study, we evaluated lesion formation in atherosclerosis-prone low-density lipoprotein receptor deficient (Ldlr) mice lacking CDGI or P2Y12 in hematopoietic cells. APPROACH AND RESULTS—Lethally irradiated Ldlr mice were reconstituted with bone marrow from wild-type (WT), Caldaggef1 (cdgI), p2y12, or cdgIp2y12 (double knockout DKO) mice and fed a high-fat diet for 12 weeks. Ldlr chimeras deficient for CDGI or P2Y12 developed significantly smaller atherosclerotic lesions in the aortic sinus and in aortas when compared with the Ldlr/WT controls. We also observed a significant reduction in platelet-leukocyte aggregates in blood from hypercholesterolemic Ldlr/cdgI and Ldlr/p2y12 chimeras. Consistently, fewer macrophages and neutrophils were detected in the aortic sinus of Ldlr/cdgI and Ldlr/ p2y12 chimeras. Compared with controls, the plaque collagen content was significantly higher in Ldlr chimeras lacking CDGI. Interestingly, no statistically significant additive effects were seen in Ldlr/DKO chimeras when compared with chimeras lacking only CDGI. CONCLUSIONS—Our findings suggest that CDGI is critical for atherosclerotic plaque development in hypercholesterolemic Ldlr mice because of its contribution to platelet-leukocyte aggregate formation and leukocyte recruitment to the lesion area.