In the present study, we investigated the role of the CD40L-CD40 pathway in a model of progressive atherosclerosis. ApoE−/− mice were treated with an anti-CD40L antibody or a control antibody for 12 wk. Antibody treatment started early (age 5 wk) or was delayed until after the establishment of atherosclerosis (age 17 wk). In both the early and delayed treatment groups, anti-CD40L antibody did not decrease plaque area or inhibit lesion initiation or age-related increase in lesion area. The morphology of initial lesions was not affected, except for a decrease in T-lymphocyte content. Effects of anti-CD40L antibody treatment on the morphology of advanced lesions were pronounced. In both the early and delayed treatment groups, T-lymphocyte content was significantly decreased. Furthermore, a pronounced increase in collagen content, vascular smooth muscle cell/myofibroblast content, and fibrous cap thickness was observed. In the delayed treatment group, a decrease in lipid core and macrophage content occurred. Interestingly, advanced lesions of anti-CD40L antibody-treated mice exhibited an increased transforming growth factor β1 immunoreactivity, especially in macrophages. In conclusion, both early and delayed treatment with an anti-CD40L antibody do not affect atherosclerotic lesion initiation but do result in the development of a lipid-poor collagen-rich stable plaque phenotype. Furthermore, delayed treatment with anti-CD40L antibody can transform the lesion profile from a lipid-rich to a lipid-poor collagen-rich phenotype. Postulated mechanisms of this effect on plaque phenotype are the down-regulation of proinflammatory pathways and up-regulation of collagen-promoting factors like transforming growth factor β.