Fibrillar glomerular diseases are defined by the ultrastructural accumulation of glomerular extracellular microfibrils or microtubules. The deposits have compositions particular to specific diseases, and it is important to distinguish among these clinical pathologic entities and their differing clinical implications. Amyloidosis and immunotactoid glomerulopathy are two examples of fibrillar glomerular diseases. Amyloidosis is a systemic disease in which the fibrils stain with Congo red and contain various biochemically distinct proteins that identify diseases with differing prognostic and therapeutic implications. Immunotactoid glomerulopathy is a primary glomerular disease, and the deposits contain immunoglobulins that do not stain with Congo red. In contrast to amyloidosis, division of immunotactoid glomerulopathy based on the immunoglobulin content or ultrastructural appearance of the deposits has no clinical relevance.