In the renal biopsy samples of some patients with IgA glomerulonephritis (IgA GN), tubulointerstitial changes and a significant correlation between these changes and the serum creatinine levels had been observed earlier. In order to get an insight into the function of the tubules, 45 patients with IgA GN have been examined for proteinuria with special reference to low molecular weight (LMW) proteins, also called tubular proteins using sodium-dodecylsulphate polyacrylamide gel electrophoresis (SDS-PAGE). Thirty-seven of the 45 patients had proteinuria (200-1890 mg/day). On the basis of the middle molecular weight/high molecular weight (MMW/HMW) protein ratio, the proteinuria was non-selective in 28. Twenty-nine patients had 40-200 mg LMW protein/day in the urine. There was a statistically significant correlation between the tubulointerstitial changes seen in the renal biopsy samples (characterized by the tubulointerstitial index) and the tubular proteinuria. On the basis of these results it is suggested that in most patients with IgA GN there is, in addition to the glomerular lesion, also morphological and functional tubulointerstitial damage, which is in connection with the progression of the disease.