Progression to chronic renal failure is influenced by the extent of irreversible interstitial fibrosis. For a quantitative evaluation of interstitial fibrosis, we attempted color extraction of the interstitial area treated with Masson-Trichrome stain using an image analyzer. The subjects were 20 cases of IgA nephropathy with a mean age of 33.4 +/- 12.3 years. The interstitial lesions were graded simultaneously by renal pathologists without clinical information. In addition, immunohistochemical studies using specific antibodies against type III collagen, alpha-smooth muscle actin and CD34 were performed to further characterize the interstitial lesions. The percentage of interstitium in the renal cortex evaluated by an image analyzer was significantly correlated with the pathological grade of fibrous interstitial damage(sigma = 0.861, p < 0.0002), the type III collagen-positive area(sigma = 0.954, p < 0.0001) and alpha-smooth muscle actin-positive area(sigma = 0.686, p = 0.0005). Moreover, the percentage of interstitium was negatively correlated with 24-hr creatinine clearance(r = -0.744, p < 0.0001) and the numbers of peritubular capillaries detected by CD34(r = -0.649, p < 0.0014). These findings suggest that color extraction of the interstitial area stained with Masson-Trichrome using image analysis is useful for a quantitative evaluation of the interstitial fibrous lesions in IgA nephropathy.