This experimental and clinical study examined the relationship between oesophageal damage and optimal pH of refluxate, and whether 24-h pH monitoring at optimal pH could discriminate the severity of oesophagitis. The rabbit oesophagus was perfused in vivo with pepsin and hydrochloric acid solutions of differing pH for 60 min. Maximal oesophageal damage coincided with peptic solutions at pH 1.5-2.5. Fifty-nine patients with proven gastro-oesophageal reflux disorders were tested for manometric features of the lower oesophageal sphincter (pressure, overall length and abdominal length) and oesophageal exposure to different pH levels. They were classified into four groups according to the endoscopically demonstrated severity of oesophagitis. Supine exposure at the optimal pH level for pepsin activity (pH 1.5-2.5) and overall sphincter length were found to discriminate the severity of oesophagitis reliably in 75 per cent of cases.