An esophageal origin of noncardiac chest pain is generally accepted if prolonged pH and pressure recordings show that the pain episodes correlate in time with acid reflux, esophageal motor abnormalities, or a combination of both, or if provocative testing (acid perfusion, edrophonium, balloon distention) is positive. Many patients with noncardiac chest pain of esophageal origin are said to have an irritable esophagus. Irritable esophagus has been defined in two ways. Some researchers suggest it is actually a lowered esophageal pain threshold, based on the finding that such patients feel chest pain at lower balloon volumes than controls during intraesophageal balloon distention; they are said to be hypersensitive to balloon distention. Hypersensitivity to an esophageal stimulus is generally found in patients with noncardiac chest pain of esophageal origin, and hypersensitivity to a single stimulus is one criterion for a diagnosis. Our group defines irritable esophagus as a condition in which several different stimuli result in the same type of chest pain. Accordingly, we have grouped patients with esophageal chest pain into three categories: (a) patients with an acid-sensitive esophagus, in whom spontaneous pain episodes can be related to acid reflux (with or without accompanying motor disorders), and/or the acid perfusion test is positive; (b) patients with a mechano-sensitive esophagus, in whom the spontaneous pain episodes can be related to motility disturbances (without reflux), and/or the edrophonium test or balloon distention test is positive; (c) patients with an irritable esophagus, in whom some spontaneous pain episodes are related to reflux, while others are related to abnormal motility (without reflux). The last group includes patients whose spontaneous chest pain is related to reflux, with a positive motility tests; whose pain is related to abnormal motility, with a positive reflux test; and patients with positive tests for both reflux and abnormal motility. Seven studies examined a total of 281 noncardiac chest pain patients using prolonged pH and pressure recordings and provocative tests. An acid-sensitive, a mechano-sensitive, or an irritable esophagus was found in 20%, 14%, and 24% of patients, respectively.