This study was undertaken to investigate the effect of azuki bean (Vigna angularis) seed coats (ABSC), which contain polyphenols, on the infiltration of macrophages and the progression of diabetic nephropathy in streptozotocin (STZ)-induced diabetic rats. The diabetic rats were divided into three groups with 0% (commercial diet), 0.1% and 1.0% ABSC diets. The vehicle-injected controls were given a commercial diet. At 10 weeks, the macrophage kinetics, the degree of fibrosis in glomeruli and mRNA expression for monocyte chemoattractant protein-1 (MCP-1) were examined. There was no difference in plasma glucose levels between diabetic rats treated with and without ABSC. The plasma levels of malondialdehyde (MDA) in the ABSC-treated diabetic rats were significantly lower than those in the untreated diabetic rats. Histopathologically, the percentage of the fibrotic areas stained by Sirius red stain in the glomeruli in the ABSC-treated diabetic rats was lower than in the untreated diabetic rats. ED1-positive macrophages in the glomeruli and tubulointerstitium in the untreated diabetic rats showed a significant increase in number compared with the controls. In contrast, the number of macrophages in the ABSC-treated diabetic rats was smaller than that in untreated diabetic rats. MCP-1 mRNA expression, estimated by real-time quantitative RT-PCR, was increased 2.5-fold in the untreated diabetic rat kidney, while a lower level was observed in the ABSC-treated diabetic rats. In conclusion, our results suggest that ABSC treatments suppress the increased number of infiltrating macrophages and MCP-1 mRNA expression, and attenuated the glomerular expansion in STZ-induced rat diabetic nephropathy.