Simultaneous ambulatory esophageal pH monitoring was performed in 10 patients (group 1) with normal distal acid exposure and in 40 patients (group 2) with pathological distal reflux. The probes were placed 5 and 10 cm above the lower esophageal sphincter to quantify variations of pH values that can be due to a displacement of pH sensor. In group 1 the median percent time with pH < 4 for total and upright monitoring periods and composite score were significantly lower at the proximal than the distal level. In group 2 all pH data were significantly lower at the proximal than the distal level. The patients with pathological reflux were subdivided into two subgroups based on endoscopic findings (mild and severe esophagitis). The patients with severe esophagitis showed a proximal acid reduction higher than in patients with mild esophagitis. Nine patients with mild esophagitis showed normal values at 10 cm, but all patients with severe esophagitis had abnormal proximal acid exposure.