The ability of cricoid pressure to prevent inflation of the stomach during mask ventilation of a patient was studied. Fifty patients were randomly allocated to either have or not have cricoid pressure applied during a three-minute period of standardised mask ventilation. Patients who had had cricoid pressure applied had less gas in the stomach after mask ventilation (P less than 0.001). However, in a subgroup of patients considered difficult to ventilate, cricoid pressure seemed less successful in preventing gastric inflation. Cricoid pressure is a useful technique for reducing gastric inflation during mask ventilation, particularly in patients who are relatively easy to ventilate.