By using stable isotope methodology, we studied the disposition of ibuprofen after the first and last dose of a 4-week regimen of 800 mg of racemic ibuprofen every 8 hr in three groups of subjects: 1) young healthy volunteers (n = 8); 2) healthy elderly volunteers (n = 14); and 3) elderly patients with creatinine clearance between 30 and 70 ml/min (n = 13). Stereoselective gas chromatography-mass spectrometry was used to quantify deuterated S- and nondeuterated R- and S-ibuprofen in serum up to 24 hr after the first and last doses. Urinary excretion of the stereoisomeric forms of carboxyibuprofen, hydroxyibuprofen and ibuprofen glucuronide were determined up to 24-hr postdose by stereoselective high-performance liquid chromatography. Stereoselective serum protein binding was determined by ultrafiltration. Both elderly groups had significantly decreased binding of S-ibuprofen compared to the young group. The S-ibuprofen pharmacokinetics were significantly different in the elderly patients with renal impairment compared to the young volunteers: the T1/2 was increased, the unbound clearance was decreased and the unbound concentration at steady state was increased. Fraction inverted was similar for all groups, but unbound clearances of glucuronidation and hydroxylation were reduced in the elderly patients with renal impairment. These results suggest that the disposition of ibuprofen enantiomers is altered in elderly persons with renal impairment; these changes may increase the risk for nonsteroidal anti-inflammatory drug-associated adverse effects in such patients.