53 of 61 patients successfully completed 24 h ambulatory monitoring of oesophageal pH. The indications were: symptoms suggestive of gastro-oesophageal reflux but with normal endoscopy (19 cases); atypical chest pain with normal endoscopy (21 cases); or respiratory symptoms possibly due to reflux (13 cases). A temporal association between abnormal reflux and the presenting symptoms was demonstrated in 25 patients (47%). 6 patients were shown to have respiratory symptoms after episodes of reflux which resolved on treatment of the reflux alone. Reflux occurring only in the erect posture was observed in some patients and may have been a manifestation of the irritable bowel syndrome. Reflux as a cause of symptoms was excluded in 14 patients. The procedure was well tolerated in most patients, simple to operate, and inexpensive.