In an attempt to clarify the Clinico-pathological characteristics of IgA nephropathy with acute onset, comparative studies among patients with IgA nephropathy with acute onset (group I) and with incidious onset (group II), and patients with Henoch-Schönlein purpura nephritis (HSPN) were carried out. Of 135 patients with immuno-pathologically proven IgA nephropathy, 12 patients, 5 males and. 7 females, ages ranging from 6 to 63 years, with a mean of 25 years, manifested an abrupt appearance of acute nephritic syndrome such as macroscopic hematuria, massive proteinuria and edema. They revealed a larger amount of urinary protein of 2.3±0.4 g/day (mean±SEM, p<0.01) including 7 nephrotic patients (58%, p<0.01) as compared to group II, which showed a urinary pro-tein level of 1.2±0.1 g/day including 9 nephrotic patients (7%). Thirty-nine patients with HSPN revealed urinary protein of 1.4±0.2 g/day and of nephrotic range in 10 patients (26%, p<0.01). Regarding the pathological findings, glomerular crescents were found in 9 patients (75%) in group I and 18 (46%) in HSPN, showing a significantly higher in-cidence compared to group II (32/123, p<0.01, p<0.05, respectively), although the grade of mesangial proliferation did not differ among the three groups. The immuno-fluorescence study disclosed IgA deposits along the glomerular capillary wall as well as in the mesangial area in all group I patients, in 45 (37%) group II patients, and in 20 (5100, ) patients with HSPN. As for the clinical courses, 3 patients died in group I within one year, but the clinical status in the other 9 patients in this group improved within 3 years. The clinical status in patients with HSPN similarly improved within 3 years with no patients getting worse, but 4 patients died in this group. On the contrary, no apparent change of clinical status was observed in group II during a 5-year follow up. These results suggest that patients of group I IgA nephropathy and HSPN flare up in the early phase of the disease, but follow an improving clinical course when having escaped from development of chronic renal failure in the early phase. This is in contrast to group II IgA nephropathy who showed no changes of clinical status during the follow-up period.