Platelet-endothelial interactions have been linked to increased inflammatory activation and a prothrombotic state in atherosclerosis. The interaction between von Willebrand factor (vWF)-A1 domain and platelet glycoprotein (GP) Ib/IX plays a significant role in mediating the adhesion of platelets to the injured endothelium. In the present study, contrast-enhanced ultrasound (CEU) molecular imaging with microbubbles bearing the vWF-A1 domain was performed to non-invasively monitor activated platelets on the vascular endothelium in the procession of atherosclerosis. A targeted CEU contrast agent was prepared by attaching the vWF-A1 domain to the shell of microbubbles (Mb(A1)). Rat isotype control antibody was used to produce control (Mb(ctrl)) microbubbles. The binding of Mb(A1) and Mb(ctrl) to activated platelets was assessed in in vitro flow chamber experiments. Apolipoprotein E (ApoE(-/-)) deficient mice were studied as a model of atherosclerosis. At 8, 16 and 32 weeks of age, CEU molecular imaging of the proximal aorta with Mb(A1) and Mb(ctrl) was performed and the imaging signals from microbubbles were quantified. Atherosclerotic lesion severity and platelets on the endothelial surface were assessed by histology and immunohistochemistry. In in vitro flow chamber studies, attachment of Mb(A1) to activated platelets on culture dishes was significantly greater than that of Mb(ctrl) across a range of shear stresses (P<0.05). The attachment of Mb(ctrl) was sparse and not related to the aggregated platelets. As lesion development progressed in the ApoE(-/-) mice, molecular imaging of activated platelets demonstrated selective signal enhancement of Mb(A1) (P<0.05 vs. Mb(ctrl)) at all ages. Selective signal enhancement from Mb(A1) increased from 8 to 32 weeks of age. Immunohistochemistry for GPIIb revealed the presence of platelets on the endothelial cell surface in each group of ApoE(-/-) mice and that the degree of platelet deposits was age-dependent. The results of the present study indicated that non-invasive CEU molecular imaging with targeted microbubbles bearing the vWF-A1 domain could not only detect activated platelets on the vascular endothelium but also indicate lesion severity in atherosclerosis.