In patients with gastro-oesophageal reflux disease, the pH of refluxed gastric contents has a direct bearing on disease severity and oesophageal damage. A pH of 4 has been defined as a threshold below which refluxed gastric contents become injurious to the oesophagus. Studies in patients with erosive oesophagitis have shown that the 8-week healing rates produced by acid suppressive therapy are related to the duration of time over a 24-h period that the intragastric pH > 4. The most effective agents, providing more hours with gastric pH > 4 and higher healing rates in erosive oesophagitis patients, are the proton pump inhibitors. The intragastric pH is often used to gauge the efficacy of acid suppressive therapies. There are several factors, however, that may affect the clinical relevance of such data, and these should be taken into account when comparing the effectiveness of different therapies. Firstly, because of inter-individual variation, studies comparing therapies should be of a crossover design, so that responses to therapy are measured in the same individuals. Secondly, the Helicobacter pylori status of the individual should be known, as H. pylori infection has been shown to increase intragastric pH readings when examining the effect of acid suppressive therapy. Other factors, such as positioning of the pH electrode and the duration of previous therapy, also need to be standardized when using intragastric pH to assess the efficacy of different therapies. Crossover studies comparing standard doses of different proton pump inhibitors have shown that esomeprazole 40 mg produces a significantly greater amount of time at intragastric pH > 4 on day 5 of treatment than standard doses of other proton pump inhibitors. The correlation between greater efficacy in acid suppression and clinical benefit is supported by the findings that esomeprazole 40 mg is the only proton pump inhibitor to provide greater 8-week healing rates in patients with erosive oesophagitis than both omeprazole and lansoprazole. These data all suggest strongly that intragastric pH monitoring is an effective surrogate marker and a relevant predictor of outcome in gastro-oesophageal reflux disease.