Objective: Tubulointerstitial fibrosis is one of the strongest independent predictive factors in determining the prognosis in IgA nephritis. Recently, software-based quantitative measurement of interstitial fibrosis with Sirius Red staining has entered the practice. The objective of this study was to investigate the prognostic value of measurement of interstitial nephritis with this method in IgA nephritis. Method: Forty-three patients diagnosed with IgA nephritis with renal biopsy between the years 2005 and 2009 were included in this retrospective observational study. The diagnostic biopsies of 37 patients were examined. Basal data included age, gender, creatinine level, glomerular filtration rate (GFR), presence of proteinuria, hypertension, glomerulosclerosis, mesangial proliferation, and interstitial fibrosis and fibrosis index calculated by the measurement of computed images of Sirius Red positive areas. Final visit included evaluation of development of end-stage renal disease (ESRD), and GFR (whether = 60 mL/min or <60 mL/min). Results: Numbers of patients with hypertension (75% vs. 34.5%; p = 0.050), ESRD development (62.5% vs. 20.7%, p = 0.035), GFR <60 mL/min (87.5% vs. 31%; p = 0.007) were greater; and basal GFR (34.25 +/- 25.29 vs. 64.14 +/- 35.34; p = 0.048) was lower in high-intensity interstitial fibrosis group (>1000 mu m2) compared to low-intensity interstitial fibrosis group (<= 1000 mu m(2)). Conclusion: Quantitative analysis of computed imaging of areas of Sirius Red positive tubulointerstitial fibrosis might serve as an effective novel method to determine the prognosis in IgA nephritis.