Studies were done to evaluate the lower oesophageal sphincter function of chronic smokers compared with non-smokers and to ascertain the acute effects of smoking on the sphincter and the occurrence of acid reflux. All subjects (non-smokers, asymptomatic cigarette smokers, and smokers with oesophagitis) were studied postprandially with a lower oesophageal sphincter sleeve assembly, distal oesophageal pH electrode, and submental electromyographic electrodes. The two groups of cigarette smokers then smoked three cigarettes in succession before being recorded for an additional hour. As a group, the cigarette smokers had significantly lower lower oesophageal sphincter pressure compared with non-smokers but the sphincter was not further compromised by acutely smoking cigarettes. Cigarette smoking did, however, acutely increase the rate at which acid reflux events occurred. The mechanisms of acid reflux during cigarette smoking were mainly dependent upon the coexistence of diminished lower oesophageal sphincter pressure. Fewer than half of reflux events occurred by transient lower oesophageal sphincter relaxations. The majority of acid reflux occurred with coughing or deep inspiration during which abrupt increases in intra-abdominal pressure overpowered a feeble sphincter. We conclude that cigarette smoking probably exacerbates reflux disease by directly provoking acid reflux and perhaps by a long lasting reduction of lower oesophageal sphincter pressure.