We prospectively studied renal tubular function during 11 consecutive exacerbations of proliferative glomerulonephritis in 8 patients with systemic lupus erythematosus (SLE). We found a rise in the fractional excretion of beta 2-microglobulin (p less than or equal to 0.05) and dimercaptosuccinic acid (DMSA; p less than or equal to 0.02) during the exacerbations. These changes coincided with a fall in the glomerular filtration rate (p less than or equal to 0.02). Since fractional excretion of beta 2-microglobulin and DMSA can be considered as markers for tubular function, their rise during exacerbation and their fall (p less than or equal to 0.01) to control values during remission indicate that tubular dysfunction frequently occurs during active proliferative glomerulonephritis in SLE and can be influenced by immunosuppressive treatment. As no correlation was found between the different tubular function studies and the activity index of tubulointerstitial abnormalities in the renal biopsy, it is suggested that tubular function studies are probably a more sensitive indicator of tubulointerstitial disease than this activity index.