To investigate the pathogenetic mechanisms of mesangial interposition (MI) in IgA nephropathy, we examined renal biopsy samples from 20 patients with IgA nephropathy. Electron microscopic morphometric analysis showed that cytoplasmic protrusion of mesangial cells (MCs) into endothelial cells (ECs) or capillary lumina, and widening of the lamina rara interna (LRI) were significantly more prominent in glomeruli with MI than in those without. Immunoelectron microscopy revealed that the ratio of positive endothelial staining with a polyclonal antibody against human platelet-derived growth factor (PDGF) BB was significantly higher in capillary loops with MI than in those without. Enhanced capillary staining of fibronectin related to MI was not recognized. These results suggest that MI in IgA nephropathy is caused by factors such as enhancement of cytoplasmic extensibility of the MCs, widening of the LRI and chemotactic influence of PDGF-BB located in the glomerular ECs.