BACKGROUND: It is estimated that up to 70% of community-based patients with heartburn have no evidence of esophageal mucosal injury or non-erosive gastroesophageal reflux disease (NERD). Studies in NERD patients using esophageal pH monitoring and symptom index (SI) have suggested different subgroups or clinical forms. OBJECTIVE: Our aim was to investigate frequency of different subgroups of patients with NERD according to esophageal pH-monitoring parameters and SI, and to determine esophageal acid exposure characteristics and the relationship between symptoms and acid reflux in NERD. MATERIAL AND METHODS: Patients with heartburn at least 3 times per week during 12 weeks during the last year and negative upper GI endoscopy were studied. All patients were submitted to 24 h esophageal pH-metry and SI was calculated. Patients were classified into three groups: group I, patients with abnormal pH-metry; group II, patients with normal pH-metry and positive SI (> or = 50%), and group III, patients with normal pH-metry and negative SI (< 50%). Total number of reflux episodes, length of episode, % time pH < 4, and Johnson-DeMeester score were evaluated. RESULTS: Three hundred patients, 206 women and 94 men, were included. A total of 137 (46%) were classified in group I, 62 (21%) in group II, and 101 (33%) in group III. There were no significant differences in gender and age among groups. All pH parameters were significantly higher in group I. In group II, there was good correlation between heartburn and acid reflux. Esophageal acid exposure in group III was minimal and there was no relationship among symptoms and acid reflux episodes. CONCLUSIONS: Subjects with NERD represent a heterogeneous group of patients. Three subgroups of patients with NERD were identified by pH-metry and SI; a) patients with abnormal acid exposure; b) patients with hypersensitive esophagus (heartburn with physiologic amounts of acid reflux), and c) patients with no correlation between symptoms and acid reflux.