BACKGROUND Angiotensin II (Ang II)-induced cardiac remodeling with the underlying mechanisms involving inflammation and fibrosis has been well documented. Cytosolic adaptor caspase recruitment domain 9 (CARD9) has been implicated in the innate immune response. We aimed to examine the role of CARD9 in inflammation and cardiac fibrosis induced by Ang II. METHODS Two-month-old CARD9-deficient (CARD9(-/-)) and wild-type (WT) male mice were infused with Ang 11 (1,500 ng/kg/min) or saline for 7 days. Heart sections were stained with hematoxylin and eosin and Masson trichrome and examined by immunohistochemistry; and activity and protein levels were measured in macrophages obtained from mice. RESULTS WT mice with Ang II infusion showed a marked increase in CARD9(+) macrophages in the heart, but CARD9(-/-) mice showed significantly suppressed macrophage infiltration and expression of proinflammatory cytokines, including interleukin-1 beta (IL-1 beta) and connective tissue growth factor (CTGF). Importantly, Ang II-induced cardiac fibrosis (extracellular matrix and collagen I deposition) was diminished in CARD9(-/-) hearts, as was the expression of transforming growth factor-beta (TGF-beta) and level of myofibroblasts positive for a-smooth muscle actin (alpha-SMA). Furthermore, Ang II activation of nuclear factor-kappa B (NF-kappa B), JNK and p38 mitogen-activated protein kinases (MAPKs) in WT macrophages was reduced in CARD9(-/-) macrophages. CONCLUSION CARD9 plays an important role in regulating cardiac inflammation and fibrosis in response to elevated Ang II.