Gastroesophageal reflux disease (GERD) is defined as 'Chronic symptoms or mucosal damage produced by the abnormal reflux of gastric contents into the esophagus'. Reflux esophagitis refers to a subgroup of GERD patients with histopathologically demonstrated characteristic changes in the esophageal mucosa. Besides, GERD includes symptoms without endoscopic findings (endoscopic negative GERD) and extra-esophageal symptoms. Therefore, GERD cannot be diagnosed only by endoscopy. Three methods are indispensable in the diagnosis of GERD; endoscopy, evaluation of patient symptoms and acid reflux. Since 'Symptom relief is well correlated with the degree or suppression of gastric acid secretion in GERD', symptom in relation to acid reflux can be evaluated by PPI-test. Characteristics of PPI-test including extra-esophageal GERD diagnosis are discussed in this review.