Background: We previously demonstrated that vascular injury-induced neointima formation is exaggerated in human C-reactive protein (CRP) transgenic (CRPtg) compared to nontransgenic (NTG) mice. We now test the hypothesis that complement is required for this effect. Methods and results: CRPtg and NTG with a normal complement system versus their counterparts lacking expression of complement component 3 (C3) protein (CRPtg/C3-/- and NTG/C3 -/-) underwent carotid artery ligation. Twenty-eight days later, the injured vessels in CRPtg had thicker neointimas and more immunoreactive C3 in the surrounding adventitia compared with NTG. In CRPtg/C3-/-, there was no increase in neointimal thickness compared with NTG or NTG/C3 -/-. Decreasing human CRP blood levels through administration of a selective antisense oligonucleotide eliminated the depletion of serum C3 associated with vascular injury and reduced immunoreactive C3 in the resultant lesions. In injured vessels, C3 colocalized with F4/80 (macrophage marker), and in vitro, human CRP elicited increased expression of C3 by bone marrow-derived macrophages. Conclusion: Human CRP exaggeration of neointima formation in injured mouse carotid arteries associates with decreased circulating C3 and increased tissue-localized C3. C3 elimination or pharmacological reduction of human CRP prevents CRP-driven exacerbation of the injury response. In the CRPtg model system, mouse C3 is essential for the effect of human CRP. © 2010 American Heart Association, Inc.