The adoption of specific criteria for the reading of tracings, together with a comparison of the results obtained during the course of patient tests with the thresholds of normality calculated in groups of healthy volunteers, are required to determine the normality or abnormality of pH monitoring. From such comparisons performed among groups of healthy volunteers, selected especially on the basis of age or nationality, it was found that age bears no significant influence in calculating the parameters pertinent to pH monitoring. On the other hand, different dietary and life habits could be responsible for a few, albeit limited, discrepancies, such as, for instance, the number of recorded occurrences of gastro-esophageal acid reflux. Use of esophago-gastric pH monitoring, which makes it possible to identify both acid and non-acid gastro-esophageal reflux (mixed and alkaline), authorizes the diagnosis of that limited number of patients in whom this latter component of reflux only exceeds normal limits (3.3%) and an improved definition of the clinical picture in a larger share of patients (20%). Use of different statistical methods to calculate the thresholds of normalcy does not substantially improve the sensitivity of the examination, changing the thresholds by a few tenths of a unit and the fact that the examination is slightly over, or slightly under, the threshold being of ineffectual clinical significance. The study of the correlation between symptoms and pH monitoring events seems a valid interpretative criterion of these tests and capable of improving the diagnostic efficiency of the examination (0.88) when combined with mathematical evaluation.