We report here a case of fibrillary glomerulonephritis arising in a 43-year-old man with a polyclonal gammopathy, who presented with progressive renal insufficiency, microscopic hematuria, and mild proteinuria (0.7 g/d). Ultrastructural studies showed deposits of randomly oriented fibrils in the glomerular mesangium and adjacent portions of some glomerular basement membranes, with a mean fibril thickness of 14.3 nm, highly consistent with fibrillary glomerulonephritis. The Congo red stain was negative on histologic sections. Immunofluorescence studies revealed strong mesangial and focal glomerular capillary staining for immunoglobulin (Ig) G, complement (C) 3, and kappa light chains, with minimal staining for IgA, IgM, C1q, or lambda light chains. The IgG present was entirely of the IgG1 subclass. This case is quite unusual for fibrillary glomerulonephritis, which typically presents with polyclonal IgG deposits and IgG4 as the dominant IgG subclass present. Monoclonal deposits are more frequently associated with immunotactoid glomerulopathy, characterized ultrastructurally by microtubule-like structures 30 to 50 nmn thick, often in parallel arrays. The present case illustrates that although fibrillary glomerulonephritis and immunotactoid glomerulopathy might be distinguishable on ultrastructural grounds, there is overlap between these two entities with respect to the potential composition of the glomerular deposits present.