Peritoneal fibrosis (PF) is an important complication of peritoneal dialysis (PD) therapy that often occurs in association with peritoneal high transport rate and ultrafiltration failure (UFF). To study the possible pathogenic role of connective tissue growth factor (CTGF) in the relationship of PF and UFF, dialysate CTGF contents (n = 178) and tissue CTGF expression (n = 61) were investigated by ELISA, real-time PCR, immunohistochemistry, and in situ hybridization. CTGF production with and without TGF-beta1 stimulation in human peritoneal mesothelial cells (HPMC) from the spent patients' peritoneal dialysate (n = 32) was studied in vitro. The dialysate-to-plasma ratio for creatinine (D/P Cr) was positively correlated to dialysate CTGF concentration and estimated local peritoneal production of CTGF. CTGF mRNA expression was 11.4-fold higher in peritoneal membranes with UFF than in pre-PD renal failure peritoneum and was correlated with thickness of the peritoneum. CTGF protein and mRNA were detected in mesothelium and in fibroblast-like cells. In cultured HPMC, TGF-beta(1)-induced expression of CTGF mRNA was increased at 12 and 24 h and was correlated with D/P Cr. In contrast, bone morphogenic protein-4 mRNA expression was inversely correlated with D/P Cr. Our results suggest that high peritoneal transport state is associated with fibrosis and increased peritoneal CTGF expression and production by mesothelial cells, which can be stimulated by TGF-beta1. Dialysate CTGF concentration could be a biomarker for both peritoneal fibrosis and membrane function. Functional alteration of mesothelial cells may be involved in progression of peritoneal fibrosis in high transport state.