Patterns of lower oesophageal sphincter (LOS) function associated with the onset of 644 reflux episodes were recorded and analysed in 67 patients referred for evalution of gastrooesophageal reflux (GOR). Patients were studied recumbent, for one hour before and four hours after a standard meal. Transient LOS relaxation was the most prevalent mechanism and overall accounted for 82% of reflux episodes. With increasing severity of oesophagitis, absent basal LOS pressure became a progressively more common mechanism, accounting for 23% of episodes in the patients with severe oesophagitis. Patients commonly exhibited more than one mechanism. The timing of most (69%) LOS relaxations associated with reflux was not compatible with triggering by swallowing. Prolonged transient LOS relaxations were associated with inhibition of oesophageal peristalsis suggesting that this response is produced by neural inhibition. This study suggests the primary importance of transient LOS relaxations as the cause of GOR across the spectrum of severity of reflux disease.