Background: Transglutaminase (Tg)-2 is shown to be related to renal fibrosis. However, its roles in human kidney disease have not been fully studied. Methods: Using immunohistochemistry, we examined Tg-2 expression in renal biopsy specimens from 22 patients with IgA nephropathy (IgAN) and correlated the intensity of Tg-2 staining with clinical and histopathological parameters. We compared the distribution and intensity of Tg-2 staining with those of transforming growth factor (TGF)-β staining. Results: In normal human kidneys, Tg-2 staining was not significant. In IgAN kidneys, glomerular Tg-2 staining correlated with serum creatinine (S-Cr), creatinine clearance (Ccr), urinary protein excretion, glomerular sclerosis, and mesangial cell proliferation. Tubulointerstitial Tg-2 correlated with S-Cr, Ccr, N-acetyl-β-glucosaminidase, urinary β2-microglobulin, and tubulointerstitial injuries. Tg-2 staining in the vicinity of vascular poles of glomeruli preceded the development of mesangial lesions, and was more remarkable in cases with renal impairment. The distribution and intensity of Tg-2 staining were not consistent with those of TGF-β staining. In glomerular crescents, Tg-2 staining was remarkable. Conclusion: The present study showed a correlation between Tg-2 expression and renal function and pathological changes. Tg-2 expression in the vicinity of vascular poles was notable because that may be an initial marker of glomerular injury. Copyright © 2007 S. Karger AG.