Recently, the authors reported that the ratio of serum IgA to C3 (serum IgA/C3 ratio) is a good marker to distinguish patients with IgA nephropathy from non-IgA nephropathy patients together with serum IgA levels using an international reference preparation (IFCC/CRM470). In this study, the authors investigated whether the serum IgA/C3 ratio might be an indicator of prognostic grading in patients with IgA nephropathy. Two hundred and thirteen patients with IgA nephropathy and 96 other glomerular diseases including diffuse or focal mesangial proliferative glomerulonephritis without mesangial IgA deposition (non-IgA PGN), membranous nephropathy and thin basement membrane syndrome were examined. The levels of serum IgA and C3 in these patients were adjusted by the specified formula to those using international standard serum (IFCC/CRM470) in this study. The results of this study showed the highest levels of IgA/C3 ratio in patients with IgA nephropathy. The serum IgA/C3 ratio appears to gradually increase according to the prognostic grading of this disease. Therefore, measurement of the serum IgA/C3 ratio may be useful for prediction of diagnosis and prognostic grading in patients with IgA nephropathy.