Prospective multicenter study involving six clinical institutions (see collaborators) with recruitment of consecutive patients who undergo routine upper endoscopy for several (non-selected) reasons. The patients will have to fill out a questionnaire about their history of reflux symptoms, including questions regarding smoking and drinking habits. The treating physician will have to complete a clinical protocol which includes a questionnaire regarding the patient's drug history. The endoscopic examination is done in a routine, yet standardized fashion. In particular, biopsy material will be taken from the stomach, the gastroesophageal junction and from the duodenum. The samples will be routinely sent to the participating institutes of pathology (see collaborators) and will be assessed by independent histopathologists with special expertise in gastrointestinal pathology. Endoscopic findings are documented in a standardized clinical protocol. Likewise, the histopathological assessment of the samples is done following a standardized protocol following the WHO classification and the updated Sydney classification, as appropriate.