To search for biomarkers of IgA nephropathy, protein profiles of urine samples from patients with IgA nephropathy and normal volunteers were compared using 2D-DIGE. Most of the 172 spots identified in the urine were serum proteins, and their amounts in IgA nephropathy urine were much higher than those in normal urine, which can be explained as proteinuria caused by glomerular dysfunction. However, only a1-microglobulin, also one of the major serum proteins, in IgA nephropathy urine was not higher in amount than that in normal urine. We confirmed using ELISA analysis that the amounts of transferrin and albumin in IgA nephropathy and diabetic nephropathy urine were much higher than those in normal urine, whereas the amount of a1-microglobulin in IgA nephropathy urine was not higher than that in normal urine and was much lower than that in diabetic nephropathy urine. Approximately 50% of a1-microglobulin forms a complex with IgA in serum. These results suggest that a1-microglobulin in IgA nephropathy urine is a characteristic protein and might be a biomarker for IgA nephropathy, and that a1-microglobulin might have a relationship with IgA nephropathy pathology.