The resistance of the lower esophageal sphincter to reflux of gastric juice is determined by the integrated effects of radial pressures exerted over the entire length of the sphincter. This can be quantitated by three-dimensional computerized imaging of sphincter pressures obtained by a pullback of radially oriented pressure transducers and by calculating the volume of this image, in other words, the sphincter pressure vector volume. Validation studies showed that sphincter imaging based on a stepwise pullback of a catheter with four or eight radial side holes is superior to a rapid motorized pullback. Compared with 50 healthy volunteers, the total and abdominal sphincter pressure vector volume was lower in 150 patients with increased esophageal acid exposure (p less than 0.001) and decreased with increasing esophageal mucosal damage (p less than 0.01). Calculation of the sphincter pressure vector volume was superior to standard techniques in identifying a mechanically defective sphincter as the cause of increased esophageal acid exposure, particularly in patients without mucosal damage. The Nissen and Belsey fundoplication increased the total and intra-abdominal sphincter pressure vector volume (p less than 0.001) and normalized the three-dimensional sphincter image. Failure to do so was associated with recurrent or persistent reflux. These data indicate that three-dimensional imaging of the lower esophageal sphincter improves the identification of patients who would benefit from an antireflux procedure. Analysis of the three-dimensional sphincter pressure profile should become the standard for evaluation of the lower esophageal sphincter.