In order to clarify the difference of clinical and pathological features between the IgA nephropathy patients with acute and insidious onset, 427 patients were examined in this study. Seventy-eight patients with acute onset (group 1) were often associated with mucosal system infections at the abrupt onset. This group revealed macroscopic hematuria, more severe microscopic hematuria (more than 20/hpf), higher glomerular filtration rate (p < 0.01) and lower serum levels of C3 (p < 0.01). It had also a significantly higher incidence of exudative lesions (p < 0.001). On the other hand, the onset of 349 patients (group 2) was noticed to be insidious without preceding infections. This group showed a more severe increase in mesangial cells (p < 0.01) and a significantly higher incidence of adhesion, arterial sclerosis and tubulointerstitial changes. Deposition of Clq, C4 and IgM and detachment of visceral epithelium from the basement membrane were more frequently seen in group 2. Twenty-seven of 345 patients followed for at least 1 year after the biopsy were on maintenance hemodialysis: 1 patient was in group 1 and 26 were in group 2. These results clarified that there was a difference in clinical, laboratory and histopathological findings between the patients with IgA nephropathy with acute and insidious onset.