Renal biopsy specimens from 163 patients with IgA nephropathy were examined by electron microscopy to clarify the significance of gaps of the glomerular basement membrane (GBM) in IgA nephropathy. Gaps of the GBM were observed in 21 cases. In 1 case the capillary lumen was partially filled with fibrin-like materials, basement membrane-like materials, and epithelial cells, and in 6 cases wide gaps of the GBM were observed. In 10 cases the gaps of the GBM were covered by epithelial and/or endothelial cells, and in 4 cases spherical microparticles were seen at the gaps of the GBM. Microscopic hematuria of less than 5 red blood cells per high-power field of vision at the time of biopsy was less frequently observed in patients with gaps of the GBM than in patients without (p less than 0.05). Marked local thinning or splitting of the GBM and electron-dense deposits or spherical microparticles on peripheral capillary walls were more frequently observed in patients with gaps of the GBM than in those without (p less than 0.001, less than 0.005, less than 0.05, and less than 0.001, respectively).