Heparin cofactor II (HCII), a serine protease inhibitor, inhibits tissue thrombin action after binding with dermatan sulfate proteoglycans in the extracellular matrix of the vascular system. We previously reported that heterozygous HCII-deficient (HCII(+/-)) humans and mice demonstrate acceleration of vascular remodeling, including atherosclerosis. However, the action of HCII on cardiac remodeling never has been determined. HCII(+/+) and HCII(+/-) mice at age 25 weeks were infused with angiotensin II (Ang II; 2.0 mg/kg/d) for 2 weeks by an osmotic mini-pump. Echocardiography revealed acceleration of cardiac concentric remodeling in HCII(+/-) mice and larger left atrial volume in HCII(+/-) mice than in HCII(+/+) mice. Histopathologic studies showed more prominent interstitial fibrosis in both the left atrium and left ventricle in HCII(+/-) mice than in HCII(+/+) mice. Daily urinary excretion of 8-hydroxy-2'-deoxyguanosine, a parameter of oxidative stress, and dihydroethidium-positive spots, indicating superoxide production in the myocardium, were markedly increased in Ang II-treated HCII(+/-) mice compared to those in HCII(+/+) mice. Cardiac gene expression levels of atrial natriuretic peptides and brain natriuretic peptides, members of the natriuretic peptide family, Nox 4, Rac-1, and p67(phox) as components of NAD(P)H oxidase, and transforming growth factor-beta1 and procollagen III were more augmented in HCII(+/-) mice than in HCII(+/+) mice. However, administration of human HCII protein attenuated all of those abnormalities in Ang II-treated HCII(+/-) mice. Moreover, human HCII protein supplementation almost abolished cardiac fibrosis in Ang II-treated HCII(+/+) mice. The results indicate that HCII has a protective role against Ang II-induced cardiac remodeling through suppression of the NAD(P)H oxidase-transforming growth factor-beta1 pathway.