Kidney biopsies from 12 patients between the ages of 10 and 63 yr were diagnosed as nonamyloidotic fibrillary glomerulopathy (NAFG) or immunotactoid glomerulopathy (IG) on the basis of the electron microscopic finding of filamentous or tubular material within the glomerular capillaries and mesangium. Six patients were male and six were female. Eleven presented with nephrotic syndrome and one with acute renal failure. Eight were hypertensive, and four of these patients had gross or microscopic hematuria as well. Biopsies from 11 patients were Congo red negative; one was weakly positive. By light microscopy, the predominant glomerular change was thickening of the capillary basement membrane with or without widening of the mesangium; these changes were suggestive of membranous glomerulonephritis. Immunofluorescent studies performed in four of the cases were positive for immunoglobulin G (IgG). Immunoperoxidase staining for beta 2-microglobulin was negative in four patients. Ultrastructurally, filaments or tubules were identified in the glomerular capillary basement membrane and/or mesangium in each patient. The filaments in NAFG, IG, amyloidosis, and other paraprotein deposits can be differentiated by size, arrangement, and location of filamentous material.