BACKGROUND: Studies have demonstrated that patients with Barrett's oesophagus have the highest oesophageal acid exposure profile, followed by erosive oesophagitis and non-erosive reflux disease patients, but the exact extent of overlap remains unknown. AIM: To determine the extent of overlap in oesophageal acid exposure among the different gastro-oesophageal reflux disease groups. METHODS: A total of 121 patients with gastro-oesophageal reflux disease underwent an upper endoscopy and were classified as having Barrett's oesophagus, erosive oesophagitis and non-erosive reflux disease-all (non-erosive reflux disease-positive and functional heartburn). Subsequently, patients underwent pH testing and overlap in oesophageal acid exposure among the different gastro-oesophageal reflux disease groups was determined. RESULTS: Of those enrolled, 24 had Barrett's oesophagus, 30 erosive oesophagitis and 28 were non-erosive reflux disease-positive. Mean oesophageal acid exposure time was 224.8 +/- 35, 134.3 +/- 21.9 and 141.3 +/- 19.8 min for Barrett's oesophagus, erosive oesophagitis and non-erosive reflux disease-positive respectively. Per cent overlap for total, upright and supine time between non-erosive reflux disease-positive and erosive oesophagitis was 47.4%, 64.7% and 81.8%, between Barrett's oesophagus and erosive oesophagitis was 47.8%, 40.7% and 24%, and between Barrett's oesophagus and non-erosive reflux disease-positive was 31.6%, 37.5% and 20.8% respectively. CONCLUSIONS: Our study demonstrated a high oesophageal acid exposure overlap between patients with non-erosive reflux disease-positive and erosive oesophagitis, Barrett's oesophagus and erosive oesophagitis, as well as Barrett's oesophagus and non-erosive reflux disease-positive patients.