Esophageal pH monitoring data are influenced by numerous patient- and/or technical equipment-related factors. In this study, data obtained with 10 pH recording devices of two different companies and 10 pH glass-microelectrodes with an external reference electrode of one company, calibrated by different persons, were compared. Pearson correlation coefficients regarding the reflux index (the percentage of the investigation time with a pH less than 4) ranged from 0.93 to 1.00. The lowest correlation coefficient (0.73) was obtained by comparing the number of episodes with a pH less than 4 recorded with devices from different companies. These results suggest that differences in pH monitoring data recorded in comparable conditions with one type of pH electrode are irrelevant as far as clinical routine investigations are concerned. Data are not significantly influenced by the recording devices themselves, the age of the electrodes, or the skin-potential differences induced by the person calibrating the probes.