Many of the present definitions of Barrett's esophagus are based on the dogma that 2 to 3 cm of cardiac mucosa normally line the distal esophagus and proximal stomach. Recent autopsy data refute this dogma. Cardiac mucosa has been shown to be frequently absent from the squamocolumnar junctional zone. When present, its extent is less than 0.5 cm in almost all children and most adults. Cardiac mucosal length increases with age. Patients who have cardiac mucosa are significantly more likely to have abnormal acid exposure in the esophagus as measured by 24-hour pH studies. The length of the cardiac mucosa correlates significantly with the amount of reflux: the greater the length of the cardiac mucosa, the more reflux there is. These new data provide insights into the pathology of gastroesophageal reflux. Normalcy is defined as an esophagus lined by squamous epithelium and a stomach lined by gastric mucosa. Reflux disease is defined by the presence of cardiac mucosa in a junctional biopsy. The severity of reflux disease is quantifiable by the length of cardiac mucosa present. Mutational reflux disease (Barrett's esophagus) is defined by the occurrence of intestinal metaplasia in cardiac mucosa and is quantitated by the amount of intestinal metaplasia present. Neoplastic reflux disease is defined as the occurrence of low grade dysplasia, high grade dysplasia, and adenocarcinoma in Barrett's esophagus. An attempt is made here to develop a rational grading system for reflux based on these highly objective histologic criteria.