To evaluate the significance of tubulointerstitial lesions in IgA nephropathy, clinicopathological studies were performed on 90 cases of this disease. The degree of tubulointerstitial lesions was assessed semiquantitatively by light microscopic observation. The relation between glomerular filtration rate (GFR) and multiple clinicopathological factors was analysed by correlation analysis and logistic regression. The results showed that the degree of renal impairment at the time of biopsy was clearly correlated with and largely determined by the tubulointerstitial lesions, in particular the density of interstitial infiltration by lymphocytes and monocytes/macrophages. It is indicated that tubulointerstitial lesions play an important role in the pathogenesis of renal impairment in IgA nephropathy.