In the MUSE classification of gastroesophageal reflux disease (GERD), esophagitis is assessed by the presence of metaplasia, ulcer, stricture, or erosion, each being graded as absent, mild or severe. Daily reflux symptoms affect about 4 to 7 percent of the population; erosive esophagitis occurs in about 2 percent; the prevalence rate of Barrett's metaplasia is 0.4 percent; and esophageal adenocarcinoma leads to two deaths per million living population. In persons with GERD symptoms, about 20 percent are found to have erosive esophagitis, while ulcers or strictures are found in less than 5 percent of all patients with erosive esophagitis. No clear-cut temporal progression exists between successive grades of disease severity, as the most severe grade of GERD is reached at the onset of the disease. Mild forms of GERD tend to be more common in women than men, while severe GERD characterized by erosive esophagitis, esophageal ulcer, stricture or Barrett's metaplasia are far more common in men than women. All forms of GERD affect Caucasians more often than African Americans or Native Americans. The prevalence of GERD is high among developed countries in North America and Europe and relatively low in developing countries in Africa and Asia. During the past three decades, hospital discharges and mortality rates of gastric cancer, gastric ulcer and duodenal ulcer have declined, while those of esophageal adenocarcinoma and GERD have markedly risen. These opposing time trends suggest that corpus gastritis secondary to Helicobacter pylori infection protects against GERD. This hypothesis is consistent with the geographic and ethnic distributions of GERD. Case-control studies also indicate that cases with erosive esophagitis are less likely to harbor active or chronic corpus gastritis than controls without esophagitis.