Gastric contents regurgitation into the oesophagus during induction of anaesthesia may easily fail to be recognized. The incidence of this complication was investigated in 59 consecutive patients. They were scheduled for elective thoracic surgery. Anaesthesia was induced with thiopentone (6 mg.kg-1), fentanyl (3 micrograms.kg-1), and either atracurium (0.4 mg.kg-1), vecuronium (0.1 mg.kg-1) or suxamethonium (1.5 mg.kg-1). Oesophageal pH was monitored with an oesophageal pH probe, connected to a portable computer. The pH probe had a virtually instantaneous response time and was positioned in the lower oesophagus. Acid reflux was defined as a decrease in pH to less than 4.0. During the course of induction, three patients (5%) presented an episode of acid reflux. No patient presented any clinical or radiological signs of pulmonary aspiration. This study suggests that monitoring oesophageal pH is a simple method of detecting gastric reflux during the period of induction.