A prospective study was performed in 17 patients with achalasia of the esophagus determining the manometric characteristics of the gastroesophageal sphincter, correlating in with hisotological analysis by biopsies taken during surgery at the distal narrowed segment of the esophagus, at the location of the sphincter. The histological findings were compared to 10 control cases. Presence or absence of ganglion cells at the Auerbach's plexuses and appearance of smooth muscle fibers were evaluated. Only one case (6%) had Chagas' disease. The mean sphincter pressure was 41 mm Hg, with incomplete relaxation in all patients. Histological analysis showed a complete disappearance of ganglion cells in 94% of the cases and a decrease in the number of neurons with marked chronic inflammatory cells in one case (6%). In all control cases, the ganglion cells were normal. Smooth muscle fibers were normal on light microscopy. No relationship was found between resting gastroesophageal sphincter pressure,