The technique of 24 hour esophageal pH monitoring (24 hour pH test) is described. Experience with the 24 hour pH test in 393 patients with suspected esophageal disease has shown the clinical usefulness of the test in objectively determining the presence of gastroesophageal reflux. The test was effective in evaluating atypical symptoms of gastroesophageal reflux such as respiratory symptoms and chest pain and, in children, failure to thrive and recurrent pneumonia. The 24 hour pH test was particularly useful in evaluating patients who were referred with other abdominal or thoracic disease and had, in addition, symptoms suggestive of gastroesophageal reflux on history. The test helped to unsnarl the cause of recurrent symptoms after an esophageal myotomy for achalasia or an antireflux procedure. Of 179 patients with typical symptoms of gastroesophageal reflux, 27% had normal 24 hour test results and were subsequently diagnosed as having another cause for their symptoms. Of 146 patients who had normal findings on esophagoscopy, 54% were shown to have abnormal gastroesophageal reflux on 24 hour pH monitoring, indicating lack of sensitivity of endoscopy to detect reflux. In addition, the 24 hour pH test identified patterns of abnormal reflux and indicated those patients most at risk for development of stricture. The test is well tolerated by the patients, simple to use, and dependable when performed and read as described. The clinical use of the 24 hour pH test brings objectivity to the evaluation of exophageal disease that has hitherto not been available.