Oesophagitis severity is related to total oesophageal acid exposure, which is in turn dependent upon both the number reflux events and the rate of refluxate clearance. This study examined differences in the frequency of reflux events and the characteristics of oesophageal acid exposure in asymptomatic controls and gastro-oesophageal reflux disease (GORD) patients both with and without oesophagitis. Nine controls and 38 patients with GORD were studied. All patients underwent upper endoscopy, videofluoroscopy, determination of resting lower oesophageal sphincter pressure (LOSP) and 24 h pH monitoring. Analysis was performed with subjects grouped as controls, non-erosive GORD and oesophagitis. A second analysis was performed with subjects grouped by hiatal hernia size. Statistical comparisons were made using anovaand unpairedt-tests. Patients with oesophagitis had significantly larger hiatal hernias, greater oesophageal acid exposure, more prolonged episodes of reflux and longer acid clearance times than did controls and patients with non-erosive GORD. No significant difference was seen in the number of reflux events. Increasing hiatal hernia size was significantly associated with increasing oesophageal acid exposure, number of prolonged reflux events and prolonged acid clearance times. Oesophagitis patients have significantly greater oesophageal acid exposure than subjects with non-erosive GORD. Increased oesophageal acid exposure in oesophagitis is attributable to impaired acid clearance and the greatest impairments in refluxate clearance occur in patients with large hiatal hernias.