We studied the anatomy of the esophageal hiatus using axial CT scans in 320 patients. Normally, the diaphragmatic crura are tightly opposed and closely related to the esophagus. Widening of the esophageal hiatus is readily demonstrated by CT as a separation of the diaphragmatic crura and an increased distance between the crura and esophageal wall. The incidence of widening increases with age, lending support to the hypothesis that the abnormality is acquired. In 100 patients who also had barium studies of the upper gastrointestinal tract, a sliding hiatus hernia was found in 37/43 (86.4%) patients who had a widened esophageal hiatus shown by CT. In 26 of 37 (70.2%) patients, the hernia was demonstrated on CT as a pseudomass usually filled with contrast material lying within and/or above the esophageal hiatus. Small hiatus hernias were shown without a widened esophageal hiatus in only two patients.