Renal diseases characterized by Congo red-negative extracellular fibrillary deposits, either organized arrays of larger, microtubular fibrils (immunotactoid glomerulopathy [IT]) or smaller, randomly organized fibrils (fibrillary glomerulonephritis), have been recognized recently. The clinical significance, if any, of the distinction of these patterns has not been determined. On review of all renal biopsy specimens evaluated in a private referral renal pathology laboratory over the last 11 years, 26 cases with fibrillary glomerulonephritis pattern were identified and compared with our six most recent cases with the IT pattern. The fibrillary glomerulonephritis patients, 17 women and nine men, had an average age of 50 +/- 2 years and contributed 1% of the renal biopsy specimens examined. All patients had marked proteinuria and 16 had microscopic hematuria. Follow-up at 23 +/- 5 months in 25 of these patients revealed end-stage renal disease in 11 patients (44%) and one death due to renal failure. End-stage renal disease developed an average of 10 +/- 5 months after biopsy. One patient developed multiple myeloma. Twenty-four renal biopsy specimens showed proliferation, with crescents in seven. Immunofluorescence showed moderate to intense staining for immunoglobulin G and weaker staining for C3, in a predominantly mesangial pattern, with weaker glomerular basement membrane (GBM) staining, corresponding to electron microscopic deposit localization. In four cases, linear GBM staining by immunofluorescence corresponded to extensive subendothelial or transmembranous deposits. The average fibril diameter was 14.0 +/- 0.5 nm (range, 10.4 to 18.4 nm). Immunotactoid glomerulopathy patients (three women and three men) were significantly older, 62 +/- 2 years (P < 0.025). All had marked proteinuria, with microscopic hematuria in two patients. Associated hematopoietic diseases were present in four patients, with monoclonal proteins and/or abnormal plasma cell proliferation in three. One patient died of nonrenal causes. The remaining five patients have stable renal function at 20 +/- 5 months. Biopsy specimens showed proliferative (n = 3) or membranous-like (n = 3) patterns. Immunofluorescence showed immunoglobulin G and weaker C3 staining in a granular GBM pattern, with lesser mesangial staining. The microtubular fibril diameter was on average 43.2 +/- 10.3 nm (range, 16.8 to 90.0 nm). Thus, fibrillary glomerulonephritis and IT can be separated based on ultrastructurally distinct features. Patients with fibrillary glomerulonephritis are less likely than those with IT to have associated hematopoietic disease and also have poorer renal survival. We propose that classification based on these morphologic differences appears to have clinical significance.