The ability of the lower esophageal sphincter to protect the esophageal mucosa from exposure to gastric juice depends on its resting pressure, length exposed to abdominal pressure, and overall length. Mechanical incompetency of the sphincter can occur when one or more of these components fail. The purpose of this study was to measure manometrically the normal percentile values for these components, and to identify the point differentiating a mechanically competent from an incompetent sphincter. The results show that a mechanically incompetent sphincter can be identified by a sphincter pressure below the 2.5 percentile, an abdominal length below the fifth percentile, and an overall length below the 2.5 percentile of normal values. Sixty percent of the patients with documented increased esophageal exposure to acid gastric juice had a mechanically incompetent sphincter.