Extraesophageal manifestations of gastroesophageal reflux may be best diagnosed using ambulatory esophageal pH monitoring. This test involves the placemenmt of a thin pH probe in the esophagus, which is connected to a small box on a waistbelt. Studies are done in an ambulatory state in the patient's home and work environment. Data collected assesses acid exposure time over the circadian cycle and the relationship of symptoms to pH drops. Studies in adult asthmatics demonstrate abnormal amounts of acid reflux by 24-hour esophageal pH monitoring in >50% of patients. Likewise, large studies in patients with chronic ENT complaints find abnormal acid reflux values in 50-80% of patients. Several problems and issues with ambulatory pH monitoring still need addressing, including (1) the need for dual pH monitoring, (2) artifact and reproducibility, (3) normal values, (4) role in the initial diagnosis, and (5) role in the follow-up of poorly responding patients.