Gastroesophageal reflux disease is a common problem that frequently presents with atypical complaints including nausea, hiccups, globus sensation, chest pain, hoarseness, coughing, or various pulmonary complaints. Diagnosis may be difficult, as these patients often do not have radiographic or endoscopic evidence of esophagitis. In these difficult cases, prolonged esophageal pH monitoring provides an accurate method of quantitating acid reflux parameters and correlating symptoms with reflux episodes in an outpatient setting. Current equipment is compact, durable, and not difficult to use or extremely expensive. Data analysis, with a particular emphasis on acid-exposure time (total, upright, supine), reliably discriminates between abnormal and normal subjects but it is not a perfect "gold standard" for gastroesophageal reflux disease. Indications for esophageal pH monitoring include: (1) atypical symptoms of acid reflux with normal endoscopy, (2) typical reflux symptoms unresponsive to medical therapy, and (3) the follow-up of reflux disease after either medical or surgical therapy. This test is currently performed primarily by gastroenterologists, but we believe many other groups may find this technology helpful. To meet these expanding applications, test refinements are necessary, particularly easier methods of placing the pH probe and better standards for defining abnormal pH parameters in older patients. The future for esophageal pH monitoring is bright. This technology has the potential to do for the diagnosis of gastroesophageal reflux disease what endoscopy has done for the diagnosis of peptic ulcer disease.