A review of 430 renal biopsies from patients with various nephropathies processed by light microscopy, immunofluorescence and in part by electron microscopy revealed 82 cases with diffuse mesangial IgA deposition. Sixty-three cases appearing without signs of systemic disease were included in this study. The glomerular changes consisted of mesangial increase (31 cases mild, and 32 cases severe) and segmental-focal glomerular lesions (40 cases). Immunofluorescence revealed granular deposits of immunoglobulins and complement within the mesangium (IgG 9, IgM 25, IgA 63, C3 27, C1q 0) as well as segmentally in the wall of capillary loops (IgG 4/26, IgM 20/26, IgA 0/26, C3 16/26, C1q 7/26). Electron microscopic studies (40 cases) showed electron dense deposits in the mesangial region in all biopsies and deposits either electron dense (11 cases) or radiolucent (26 cases) at various sites along the basement membrane of capillary loops. Comparison between morphological findings and clinical data showed a significant correlation between segmental-focal glomerular lesion on the one hand, and proteinuria (p less than 0.01), immune deposits along the capillary basement membrane (p less than 0.01), and the increased mesangium (p less than 0.001) on the other. Considering the macromolecular mesangial clearing function, the segmental-focal glomerular lesions may be due to newly arrived, infection related immune complexes which may not be cleared promptly by the mesangium, because its clearing function is impaired due to IgA deposition.