There have been a few reports suggesting that mast cells may play an important role in the development of fibrosis and/or the degradation of extracellular matrix. We examined the relationship between the number of distribution of mast cells in tubulointerstitium and prognosis of patients with IgA nephropathy. Renal biopsy specimens were stained with mouse monoclonal antihuman mast cell tryptase antibody and then with aniline blue. Specimens from 45 patients with IgA nephropathy and from 5 patients with minimal-change nephrotic syndrome were used. There was a significant correlation between the number of mast cells per unit area of the whole tubulointerstitium and the degree of tubulointerstitial fibrosis or renal function. Patients with IgA nephropathy were divided into two groups (group 1: ‘minimal’ and ‘slight’; group 2: ‘moderate’ and ‘advanced’) according to the classification by a previously reported method. Mast cells were mainly observed in the fibrotic areas in group 1. In group 2, many mast cells were detected not only in the fibrotic but also in the nonfibrotic areas. It appears that the number of mast cells in the nonfibrotic areas may be one of the predictive factors for the prognosis of patients with IgA nephropathy.