Based on immunofluorescence findings, 232 patients with IgA nephropathy were classified into two groups; one consisted of 88 patients (38%) with IgA deposits in the glomerular capillary walls together with the mesangial deposits (capillary type), and the other consisted of 144 patients (62%) with deposits confined to the mesangium (mesangial type). Electron microscopic findings revealed dense deposits on the capillary walls (subepithelial, 50%; intramembranous, 65%; and subendothelial, 24%) in 37 of 46 patients with capillary type and six of 47 with mesangial type (P less than .001). Crescent formation observed in greater than or equal to 10% of glomeruli was more frequently found in patients with the capillary type (30/88, 34%) than those with the mesangial type (9/144, 6%) (P less than .01), especially higher in those with subepithelial deposits (15/26, 57%). The capillary type patients showed heavier proteinuria (1.7 +/- 0.2 g/d) than the mesangial type patients (0.6 +/- 0.1 g/d) (P less than .05). Thirteen of the 14 patients in an acute exacerbation phase, manifested by an abrupt increase in urinary protein and development of macroscopic hematuria, showed capillary type IgA deposits. The ratio of patients with normal renal function in the fifth year after apparent onset was lower in the capillary type (74.0%) than in the mesangial type patients (96.9%) (P less than .05). These findings suggest that capillary IgA deposition is closely related to clinical and histologic activities of IgA nephropathy and is considered to be an important factor responsible for the progression of the disease, possibly through crescent formation.