The reproducibility of ambulatory 24-h esophageal pH monitoring was evaluated in 16 consecutive patients by comparing the difference in two consecutive 24-h periods. The study group included 8 patients with scleroderma esophagus and 8 treated achalasia patients. The amount of reflux was expressed as the percentage of time the pH was <4.0. Both groups demonstrated excellent intrapatient reproducibility overall: 96% in scleroderma patients and 95% in those patients with achalasia. The least concordance was found in the lengths of the longest reflux event—70% when supine in scleroderma patients and 59% when upright in patients with achalasia. There was no significant difference (p>0.05) between day 1 and day 2 for either group of patients for any of the elements studied. These results indicate that intrapatient variability of gastroesophageal reflux in patients with scleroderma esophagus and treated patients with achalasia is very low and following therapeutic intervention, a high level of confidence can be placed in subsequent pH monitoring as an indicator of treatment effect.