Endoscopic-negative reflux disease (ENRD) is a heterogeneous population. There are few data regarding the prevalenece of each group and subgroups that constitutes ENRD population. To describe the prevalence of the differents groups and subgroups in ENRD. We studied consecutive patients with frequent heartburn and acid regurgitation with endoscopically normal esophageal mucosa in whom ambulatory 24-h esophageal pH monitoring was performed. According with the pH-testing data and the correlation between symptoms and the acid reflux events, the subjects were classified in two groups: non-erosive gastroesophageal reflux disease (NERD) and functional heartburn. The functional heartburn group was splited in 3 subgroups: asymptomatics during pH-monitoring, hipersensitive esophagus and those with physiologic reflux with negative association between symptoms and acid reflux events. These last subgroup was classified in: minute changes of intra-esophegeal pH (upper than 4, n = 19) and non-acid related stimuli. One hundred and eleven patients with ENRD were classified in NERD (28%, n = 31) and functional heartburn (72%, n = 80). Patients with functional heartburn were classified in asymptomatics during pH-monitoring (11%, n = 12), hipersensitive esophagus (9%, n = 10) and those with physiologic reflux with negative association between symptoms and acid reflux events (52%, n = 58). In these last subgroup 19 (17%) patients were classified as minute changes of intra-esophegeal pH (upper than 4) and 39 (35%) as non-acid related stimuli. Even most of the patients with ENRD have normal ambulatory pH-test, their symptoms are related with drops of esophageal pH in 26% of the cases. Eleven percent do not have symptoms during ambulatory pH-monitoring.