Administration of contrast agents can cause a decrease in renal function and, occasionally, end-stage renal disease. Liver-type fatty acid-binding protein (L-FABP) is an intracellular carrier protein of free fatty acids that is expressed in proximal tubules of the human kidney. Whether urinary excretion of L-FABP can predict the occurrence of contrast medium-induced nephropathy was studied. Sixty-six patients (46 men, 20 women; mean age, 60.0 years) undergoing nonemergency coronary angiography or intervention at 1 of our institutions who had a serum creatinine (Cr) level greater than 1.2 mg/dL (> 106 micromol/L) and less than 2.5 mg/dL (< 221 micromol/L) and 30 healthy volunteers (21 men, 9 women; mean age, 56.5 years) were included. Urinary L-FABP levels were measured before and after coronary angiography with the use of monoclonal antibodies. Contrast medium-induced nephropathy is defined as an increase in serum Cr level of greater than 0.5 mg/dL (> 44 micromol/L) or a relative increase of more than 25% at 2 to 5 days after the procedure. Contrast medium-induced nephropathy occurred in 13 of 66 patients (19.7%). Before angiography, urinary L-FABP levels were significantly greater in these 13 patients (contrast medium-induced nephropathy group; 18.5 +/- 12.8 microg/g Cr; range, 5.8 to 33.6 microg/g Cr) than in the remaining 53 patients (non-contrast medium-induced nephropathy group; 7.4 +/- 4.4 microg/g Cr; range, 2.8 to 13.8 microg/g Cr; P < 0.01) or healthy volunteers (5.4 +/- 4.4 microg/g Cr; range, 1.0 to 10.0 microg/g Cr; P < 0.01). The next day and 2 days after angiography, urinary L-FABP levels increased significantly to 46.8 +/- 30.5 microg/g Cr (range, 12.0 to 84.5 microg/g Cr; P < 0.01) and 38.5 +/- 28.5 microg/g Cr (range, 9.5 to 70.5 microg/g Cr; P < 0.01) in the contrast medium-induced nephropathy group, respectively. After 14 days, serum Cr returned to the baseline level, but urinary L-FABP level remained high (34.5 +/- 30.0 microg/g Cr; range, 4.0 to 68.0 microg/g Cr). However, urinary L-FABP levels in the non-contrast medium-induced nephropathy group changed little throughout the experimental period. Urinary L-FABP level can serve clinically as a predictive marker for contrast medium-induced nephropathy.