Controversy surrounds the relatedness of fibrillary glomerulonephritis (FGN) and immunotactoid glomerulonephritis (IT). To better define their clinicopathologic features and outcome, we report the largest single center series of 67 cases biopsied from 1980 to 2001, including 61 FGN and 6 IT. FGN was defined by glomerular immune deposition of Congo red-negative randomly oriented fibrils of or = 30 nm (mean, 38.2 +/- 5.7 nm). FGN comprised 0.6% of total native kidney biopsies and IT was tenfold more rare (0.06%). Deposits in FGN were immunoglobulin G (IgG) dominant and polyclonal in 96%. IgG subtype analysis in 19 FGN cases showed monotypic deposits in four (two IgG1 and two IgG4) and oligotypic deposits in 15 (all combined IgG1 and IgG4). In IT, deposits were IgG dominant in 83% and monoclonal in 67% (three IgG1 kappa and one IgG1 lambda). FGN patients were a mean age of 57 years, 92% were Caucasian, and 39% were male. At biopsy, FGN patients had the following clinical characteristics (mean, range): creatinine 3.1 mg/dL (0.5 to 14), proteinuria 6.5 g/day (0.8 to 25), 60% microhematuria, and 59% hypertension. Histologic patterns of FGN were diverse, including diffuse proliferative glomerulonephritis (DPGN) (nine cases), membranoproliferative glomerulonephritis (MPGN) (27 cases), mesangial proliferative/sclerosing (MES) (13), membranous glomerulonephritis (MGN) (four), and diffuse sclerosing (DS) (eight). The more proliferative (MPGN and DPGN) and sclerosing (DS) forms presented with a higher creatinine and greater proteinuria compared to MES and MGN. Median time to end-stage renal disease (ESRD) was 24.4 months for FGN and mean time to ESRD varied by histologic subtype: DS 7 months, DPGN 20 months, MPGN 44 months, compared to MES 80 months and MGN 87 months. There was no statistically significant effect of immunosuppressive therapy (given to 36% of FGN patients). By Cox regression (hazard ratio, confidence interval, P value), independent predictors of progression to ESRD were creatinine at biopsy [2.05 (1.55 to 2.72) P < 0.001] and severity of interstitial fibrosis [2.01 (1.05 to 3.85) P = 0.034]. Although IT had similar presentation, histologic patterns, and outcome compared to FGN, it had a greater association with monoclonal gammopathy (P = 0.014), underlying lymphoproliferative disease (P = 0.020), and hypocomplementemia (P = 0.032). FGN is an idiopathic condition characterized by polyclonal immune deposits with restricted gamma isotypes. Most patients present with significant renal insufficiency and have a poor outcome despite immunosuppressive therapy, and outcome correlates with histologic subtype. By contrast, IT often contains monoclonal IgG deposits and has a significant association with underlying dysproteinemia and hypocomplementemia. Differentiation of FGN from the much more rare entity IT appears justified on immunopathologic, ultrastructural, and clinical grounds.