This article reviews the pharmacokinetics of esomeprazole, the (S)-isomer of the proton pump inhibitor (PPI) omeprazole. Esomeprazole is the first single isomer PPI developed for the treatment of patients with acid-related diseases. In vitro experiments in human liver microsomes demonstrated that the formation of the hydroxy and 5-O-desmethyl metabolites of esomeprazole is via cytochrome P450 (CYP) 2C19, whereas that of the sulphone metabolite is via CYP3A4. The formation rate of the hydroxy metabolite from esomeprazole is lower than for (R)-omeprazole, but that of the 2 other metabolites is higher, demonstrating stereoselective metabolism. The sum of the intrinsic clearances of all 3 metabo- lites for esomeprazole was one-third of that for (R)-omeprazole, suggesting lower clearance of esomeprazole in vivo. In vivo investigations demonstrated that esomeprazole is chirally stable after administration. Esomeprazole is 97% bound to plasma proteins. In normal (extensive) metabolisers with regard to CYP2C19, esomeprazole is metabolised more slowly than omeprazole, resulting in a higher area under the concentration-time curve (AUC) after administration of the same dose. This is more pronounced after repeated administration rather than after a single dose. In poor metabolisers, the AUC is lower for esomeprazole than for omeprazole, contributing to less overall interindividual variability for esomeprazole than for omeprazole. In general, esomeprazole and omeprazole are subject to the same metabolic transformations. Almost complete recoveries were reported and the ratio between urinary and faecal excretion is about 4:1 for both compounds. The dose-dependent increase in AUC of esomeprazole with repeated administration results from a combination of decreased first-pass elimination and decreased systemic clearance. Patients with gastro-oesophageal reflux disease exhibit a pharmacokinetic pattern similar to that in healthy individuals, whereas elderly individuals exhibited a slightly lower metabolism rate. Patients with a severe deficit in their liver function had a lower rate of metabolism, as would be expected, whereas those with mild to moderate liver disease did not exhibit any alteration in the pharmacokinetics. The pharmacokinetics of esomeprazole in individuals with impaired renal function is unlikely to differ from that in healthy individuals. A slight sex difference in the pharmacokinetics of esomeprazole was demonstrated in that the AUC and peak plasma drug concentration were slightly, but not statistically significantly, higher in females than in males.