An electron-microscopic study of the glomeruli was made on 154 children with IgA nephropathy and no evidence of systemic disease, in whom immunofluorescence microscopy had shown diffuse mesangial deposition of IgA. Mesangial deposits were observed in all but eight children. Subepithelial deposits were observed in 40 children and were almost always accompanied by both mesangial and subendothelial deposits. Subepithelial deposits were significantly associated with more severe clinical presentations, a worse outcome and more severe light microscopic glomerular changes. These observations support the concept that IgA nephropathy is an immune complex disease.