To evaluate the significance of tubulointerstitial lesions in the cortical area of renal biopsy specimens, clinicopathological studies were performed on 101 cases of IgA nephropathy, 31 cases of IgA-negative (non-IgA) proliferative glomerulonephritis and 75 cases of idiopathic membranous glomerulonephritis. The degree of tubulointerstitial lesions was assessed semiquantitatively by light microscopic observation and was correlated with the several histopathological and clinical parameters at biopsy, as well as with status at final follow-up (average follow-up period: 72 months). In these three types of glomerulonephritis, the degree of tubulointerstitial lesions in the cortical area was clearly correlated with the severity of glomerular injury, the prevalence of segmental sclerosis, global sclerosis, arteriolosclerosis, decreased renal function (GFR 150/90 mm Hg) at the time of biopsy. The prevalence of stable renal function at final follow-up was statistically higher in the cases without tubulointerstitial lesions or with those whose lesions included less than 20% of the cortical area. From the above data, it was concluded that a semiquantitative evaluation of tubulointerstitial lesions in the cortex would reflect the severity of glomerular injury and also contribute to the assessment of prognosis in such primary glomerulonephritic patients.