This is a presentation of a new manometric parameter of the mechanical competence of the lower esophageal sphincter (LES), the lower esophageal sphincter vector volume (LESVV). It is determined by computer analysis of continuous-pressure measurements during constant speed pullback of a radially oriented 4- 6- or 8-channel manometry catheter across the LES. Patients were studied with this method both before aggressive medical therapy for esophagitis and before and after Nissen fundoplication. LESVV accurately predicted failure of medical therapy and success of the fundoplication. In patients with successful fundoplication, LESVV demonstrated a 100-fold increase in mechanical competence of the LES, even in the absence of increased LES pressure or length, increasing from 113 +/- 63 mm3 to 11357 +/- 3733 mm3.