The relationship between the intensities of IgA, C3c, and C9 deposition in renal glomeruli and the severity of histopathologic injuries in patients with IgA nephropathy was examined using Microscope-Photometer O1K and a computer. Percentages of glomerular adhesion to Bowman's capsules, crescent formation, and glomerular sclerosis were calculated in the renal specimens. There was a significant correlation between the intensity of each C3c and C9 deposition in glomeruli and the degree of glomerular adhesion to Bowman's capsules and crescent formation in patients with IgA nephropathy. There was no significant correlation between the intensity of C3c or C9 deposition in glomeruli and the degree of glomerular sclerosis. No relationship was found between the intensity of IgA deposition in glomeruli and the degree of histopathologic injuries. The patients with negative or trace amounts of glomerular C3c deposits showed less severe glomerular injuries. Thus, the intensity of C3c and C9 deposition in glomeruli appears to be one of the critical factors responsible for the active progression of glomerular inflammatory process in patients with IgA nephropathy.