Diabetic nephropathy is a common cause of end-stage renal disease. The administration of an oral adsorbent, AST-120, prevents the progression of chronic renal failure in uremic rats and undialyzed uremic patients. This study was designed to determine if AST-120 slows the progression of diabetic nephropathy using Otsuka Long-Evans Tokushima Fatty (OLETF) rats, a model of non-insulin-dependent diabetic mellitus. At 21 weeks of age the OLETF rats were divided into 2 groups: AST-120-administered OLETF rats (n = 7), and control OLETF rats (n = 7). LETO rats, which are genetically similar to the OLETF rats but not diabetic, were also included. After the oral administration of AST-120 for 65 weeks, renal function and pathological changes were investigated in the 3 groups. The administration of AST-120 to the OLETF rats attenuated the progression of glomerular sclerosis, interstitial fibrosis, tubular injury as well as renal dysfunction, and reduced the serum and urinary levels of indoxyl sulfate. Furthermore, AST-120 administration reduced the interstitial expression of transforming growth factor (TGF)-β<sub>1</sub> and tissue inhibitor of metalloproteinase (TIMP)-1, as well as interstitial infiltration of macrophages. The TGF-β<sub>1</sub>-stained interstitial area showed positive correlations with the interstitial fibrosis area, the number of TIMP-1-positive cells, and the number of macrophages, and showed a negative correlation with creatinine clearance. In conclusion, AST-120 reduced the interstitial expression of TGF-β<sub>1</sub> and TIMP-1, and the interstitial infiltration of macrophages, and ameliorates the progression of diabetic nephropathy in OLETF rats.