Background: Atrial fibrosis is a feature of structural remodeling in atrial fibrillation (AF). Connective tissue growth factor (CTGF) is a potent profibrotic factor, but its role of CTGF in AF is not yet fully understood. Methods and Results:Right atrial appendages were obtained from 20 patients who underwent cardiac surgery (10 with sinus rhythm, 10 with AF). The mRNA level, protein level and immunohistochemical staining of CTGF were significantly increased in AF patients. In a porcine AF model, tissue angiotensin II (Ang II) and CTGF levels were significantly upregulated in both atria. In perfused rat hearts, Ang II stimulation increased CTGF expression, which could be inhibited by Ang II type I receptor antagonist. In a cell culture system, both atrial fibroblasts and myocytes were responsible for the increased CTGF expression under Ang II treatment. Ang II type I receptor antagonist could inhibit the Ang II-induced CTGF expression. Treating with recombinant CTGF, atrial fibroblasts expressed an increased level of collagen I. Furthermore, the CTGF level was highly correlated with tissue Ang II content in AF pigs.Conclusions: AF patients and animals exhibited a significantly increased expression of CTGF. Ang II stimulation upregulated CTGF expression in both atrial fibroblasts and myocytes. Ang II-induced CTGF expression might be involved in atrial substrate remodeling. (Circ J 2011; 75: 1592 - 1600