The segmental abnormalities of glomerular basement membrane (GBM) were studied by electron microscopy in 69 renal biopsies with acute postinfectious glomerulonephritis (APGN) and correlated with the general morphologic features and clinical findings. Thirty-six were children and 33 were adults. Biopsies were grouped into three stages by light microscopy: exudative stage (25 patients), exudative-proliferative stage (26) and proliferative stage (18). Subepithelial deposits or "humps" were present in 59 patients (86%). The frequency of humps was significantly lower at the proliferative stage than that noted in the earlier biopsies (p less than 0.01). Intramembranous, subendothelial and mesangial deposits were shown in 83% to 88% of the patients. The overall frequency of GBM abnormalities was 45%, showing significantly higher frequency in children than in adults (p less than 0.01). Dissolving subepithelial deposits were often present in the foci with GBM abnormalities. The GBM lesions were not related to more severe clinical manifestations or outcomes, but tended to occur more frequently in later biopsies (p less than 0.01). These results suggest that abnormalities of GBM in APGN are more often present than formerly assumed, especially in children, and could be a normal response to subepithelial deposits. The occurrence of these lesions in other types of immune-related glomerulonephritis may be considered along the same lines.