Tonsillectomy combined with corticosteroids has been performed for IgA nephropathy (IgAN) mainly in Japan. We, the Japanese Multicenter Study Group on the Treatment of IgA Nephropathy (JST-IgAN), have conducted a multicenter prospective cohort study including the combination therapy from 1999. A total of 101 patients (43 male, 58 female) were observed for 5 years. Their average age was 34.4 +/- 11.8 (15-55). Subjects were classified by daily proteinuria (UP) and serum creatinine (sCr); UP excreted below 0.5 g/day was defined as stage 1, 0.5-1.0 g/da y defined as stage 2, more than 1.0 g/da y and sCr < or =1.2 mg/dl in females or < or =1.4 mg/dl in males defined as stage 3, and sCr >1.2 mg/dl in females or >1.4 mg/dl in male defined as stage 4. Both tonsillectomy and high-dose corticosteroid were performed in 75 patients. Of these, 29 patients were stage 1, 26 were stage 2, 16 were stage 3, and 4 were stage 4. The number of subjects with steroid monotherapy were 1 (stage 1), 7 (stage 2), 9 (stage 3) and 1 (stage 4). The primary endpoint in this study was normalized urinalysis as clinical remission. The remission rate treated with combined therapy was 86.2% in stage 1, 73.1% in stage 2, and 43.8% in stage 3. On the other hand, it was 71.4% in stage 2, and 11.1% in stage 3 in steroid monotherapy subjects. Although the number in each treatment group varied because of the non-randomized study, tonsillectomy combined with 3 courses of high-dose corticosteroid therapy was more effective for clinical remission in the higher proteinuria group. We concluded that the goal should be cure and release from disease at an earlier stage of IgAN.