The degree of tubulointerstitial fibrosis is a poor prognostic indicator in IgA nephropathy (IgAN). Recently, connective tissue growth factor (CTGF) was observed to be strongly up-regulated in human proliferative and fibrogenic diseases. Renal biopsy specimens were obtained from the 20 patients with IgAN. Based on a previously reported study (MDRD study), all cases were categorized into 2 groups. Group A included patients with urinary protein (u-protein) or =1.0 g/day. Expressions of transforming growth factor beta1 (TGF-beta1) and CTGF mRNAs in tubular epithelial cells (TECs) were examined in all cases using rapid in situ hybridization (rISH). Significantly strong and diffuse expressions of TGF-beta1 and CTGF mRNAs were observed in proximal TECs in group B, while the expressions of these 2 mRNAs were weak in proximal TECs in group A and in glomerular resident cells in both groups. These results closely correlated with the degree of disorder in conventional pathohistological findings and clinical parameters except for u-protein level. The increase of u-protein level is one of the most important factors influencing the expression of TGF-beta1 and CTGF mRNAs in TECs. Therefore, the authors conclude that both u-protein level and expressions of these 2 mRNAs in TECs were significantly correlated with the degree of tubulointerstitial damage in IgAN.