PURPOSE: We investigated the incidence of gastroesophageal reflux disease (GERD) by 24-hour esophageal pH monitoring in patients who had chronic cough for more than 3 weeks. METHODS: From January 1995 to August 1996, 33 patients with chronic cough which had lasted for more than 3 weeks were enrolled in the 24-hour esophageal pH monitoring. The result was interpreted with the standard value which was suggested by Yvan Vandenplas in 1991 for patients younger than 15 months and with the De Meester Score for patients older than 15 months. We performed esophagoscopy and lower esophageal biopsy for patients who proved to have gastroesophageal reflux disease by 24-hour esophageal pH monitoring. RESULTS: Among 33 cases, 13 cases (39.4%) were proved to have GERD. Comparing between the normal group and the GERD group, there was no difference of gastroesophageal reflux incidence according to meal or position. We performed esophagoscopy in 10 patients among 13 GERD cases. There were 3 cases of gross esophagitis. Papillary height was elongated by more than 50% of normal in 7 cases, and basal cell layer increased by more than 20% of normal in 6 cases. Neutrophil or eosinophil infiltration was found in 2 cases, and lymphocyte infiltration was found in 7 cases. CONCLUSION: We suggest that 24-hour esophageal pH monitoring and appropriate treatment should be considered in prolonged unexplained chronic cough.