Fibrillary glomerulonephritis is a now widely recognized diagnostic entity, occurring in approximately 1% of native kidney biopsies in several large biopsy series obtained from Western countries. The distinctive features are infiltration of glomerular structures by randomly arranged fibrils similar in appearance but larger than amyloid fibrils and the lack of staining with histochemical dyes typically reactive with amyloid. It is widely but not universally recognized to be distinct from immunotactoid glomerulopathy, an entity characterized by glomerular deposits of immunoglobulin with substructural organization as microtubules and with clinical associations with lymphoplasmacytic disorders. The pathophysiologic basis for organization of the glomerular deposits as fibrils or microtubules in these entities remains obscure.