BACKGROUND Evidence indicates that urinary protein is associated with tubulointerstitial damage and thus it is an aggravating factor for chronic renal disease. As free fatty acids (FFAs) are bound to serum albumin, we hypothesized that FFAs were overloaded to the proximal tubule in massive proteinuria and thus caused tubulointerstitial damage. To test this hypothesis, massive proteinuria was provoked in mice and the renal damage examined. METHODS Mice were intraperitoneally injected with bovine serum albumin (BSA) replete with FFAs (r-BSA group, N = 10), FFA-depleted BSA (d-BSA group, N = 10), or saline (saline group, N = 9) for 14 days. RESULTS The kidneys of the r-BSA group showed severe tubulointerstitial damage and those of the d-BSA group showed mild tubulointerstitial damage. Urinary excretion of both total protein and mouse albumin were significantly higher in the r-BSA group than in the d-BSA group. To examine the proximal tubular uptake of albumin, the BSA content in the cultured mouse proximal tubules was measured by ELISA after 90 minutes of incubation with each BSA. In terms of the BSA content in the proximal tubules, there was no significant difference between the r-BSA and the d-BSA groups. These results indicate that r-BSA and d-BSA were similarly reabsorbed into the proximal tubule and that r-BSA causes severe tubulointerstitial damage. CONCLUSIONS It is the FFAs bound to albumin, rather than albumin itself, which cause severe tubulointerstitial damage by being reabsorbed into the proximal tubule. To our knowledge, this is the first in vivo observation in which FFAs have caused severe tubulointerstitial injury.