BACKGROUND Cricoid cartilage pressure induced to prevent pulmonary aspiration from regurgitation of gastric contents has been recommended, and its efficacy requires a force greater than 40 Newtons. For regurgitation to occur, both an increase in gastric pressure and relaxation of the lower esophageal sphincter (LES) are necessary. However, the effect of cricoid cartilage pressure on the LES is unknown. This study evaluated the effects of cricoid cartilage pressure on LES in human volunteers. METHODS Lower esophageal sphincter and esophageal barrier pressures (which equals LES pressure-gastric pressure) were measured using a manometric method in eight unanesthetized volunteers (4 men, 4 women) classified as American Society of Anesthesiologists physical status 1. The force applied to the cricoid cartilage was measured continuously, and LES pressure was recorded at a cricoid force of 20 and 40 Newtons. RESULTS Cricoid pressure decreased LES pressure from 24 +/- 3 mmHg to 15 +/- 4 mmHg at a force of 20 Newtons (P < 0.05) and to 12 +/- 4 mmHg with a force of 40 Newtons (P < 0.01). CONCLUSIONS These findings may explain the occurrence of pulmonary aspiration before tracheal intubation despite application of cricoid cartilage pressure.