Transforming growth factor-beta1 (TGF-beta1), an inflammatory cytokine, plays a role in tissue fibrosis, such as glomerular sclerosis and tubulointerstitial fibrosis of the kidneys. In the present study, the urinary TGF-beta1 level of cats diagnosed with chronic renal failure (CRF) was measured to investigate its relationship to the pathogenesis of feline CRF. Urinary TGF-beta1 levels (TGF-beta1/creatinine ratio) were significantly increased compared with healthy controls, whereas serum levels of TGF-beta1 were not. These results indicate that TGF-beta1 is expressed in the kidneys of CRF cats, and that it was reflected in the urinary TGF-beta1 level. Therefore, TGF-beta1 may play a role in feline CRF, and urinary TGF-beta1 could be used as a clinical marker for renal fibrosis.