If 24-hour esophageal pH monitoring is to be a useful diagnostic tool, it must reliably discriminate gastroesophageal reflux patients despite daily variations in distal esophageal acid exposure. To address this issue, we studied 53 subjects (14 healthy normals, 14 esophagitis patients, and 25 patients with atypical symptoms) with two ambulatory pH tests performed within 10 days of each other. Intrasubject reproducibility of 12 pH parameters to discriminate the presence of abnormal acid reflux was determined. As a group, the parameters of percent time with pH<4 (total,="" upright,="" recumbent)="" were="" most="" reproducible="" (80%).="" therefore,="" a="" subject="" was="" defined="" as="" having="" gastroesophageal="" reflux="" disease="" if="" at="" least="" one="" of="" these="" three="" values="" were="" abnormal.="" intrasubject="" reproducibility="" for="" the="" diagnosis="" of="" reflux="" disease="" was="" 89%="" for="" the="" entire="" sample.="" among="" subsets,="" the="" reproducibility="" was="" 93%="" for="" the="" normals="" and="" esophagitis="" patients="" and="" 84%="" for="" the="" atypical="" symptom="" patients.="" total="" percent="" time="" with=""><4 was="" the="" single="" most="" discriminate="" ph="" parameter="" (85%)="" and="" nearly="" equaled="" that="" of="" the="" three="" combined="" parameters="" (89%).="" the="" intrasubject="" variability="" of="" this="" parameter="" was="" determined="" by="" the="" mean="">sd of the relative differences between the two test results for all 53 subjects. Total percent time with pH<4 may="" vary="" between="" tests="" by="" a="" factor="" of="" 3.2-fold="" or="" less="" (218%="" higher="" to="" 69%="" lower).="" we="" conclude:="" (1)="" ambulatory="" 24-hr="" esophageal="" monitoring="" is="" a="" reproducible="" test="" for="" the="" diagnosis="" of="" gastroesophageal="" reflux="" disease;="" and="" (2)="" the="" large="" intrastudy="" variability="" in="" 24-hr="" total="" acid="" exposure="" may="" limit="" this="" test's="" usefulness="" as="" a="" measurement="" of="" therapeutic="">