From a series of 333 renal biopsies examined by immunofluorescence microscopy, 231 specimens corresponded to primary glomerulonephritis unassociated with systemic diseases. Of those 231 biopsies, 39 had diffuse mesangial deposits of IgA (16.8%). Thirty cases of IgA nephropathy had one to several glomeruli in the Epon-embedded tissue. All cases showed mild to moderate increase of mesangial cells and matrix. Mesangial deposits were present in all cases; subendothelial (5/30), intramembranous (3/30), and subepithelial (10/30) deposits wee also found. Occasional dense granular deposits involved the basement membrane of the capsule of Bowman (1/30) and the subendothelial region of some extraglomerular arterioles (2/30). Thinning (6/30) and splitting (4/30) of the glomerular lamina densa appeared focally in some cases.