The primary objective is to compare the efficacy between three doses of AZD0865 (25, 50 and 75 mg). The secondary objectives are to compare the efficacy between the three doses of AZD0865 and esomeprazole 20 mg and to evaluate the safety and tolerability of AZD0865.

Inclusion Criteria:

• Identification of their main symptom as heartburn defined as a burning feeling behind the breastbone.
• Have at least a 6-month history of heartburn and at least 4 days of episodes of heartburn with at least overall moderate intensity during the last 7 days prior to visit 1.
• Have an absence of mucosal breaks in the esophagus, as defined by the LA classification, at the visit 1 endoscopy.
• Have provided Informed consent.

Exclusion Criteria:

Subjects with current or historical evidence of the following conditions are excluded from the study:

• Erosive esophagitis.
• Esophageal columnar metaplasia (Barrett's esophagus) of 1.0 cm or greater.
• Esophageal stricture.
• Primary esophageal motility disorder(s), i.e. diffuse esophageal spasm, achalasia.
• Systemic Sclerosis (scleroderma).
• Irritable Bowel Syndrome (IBS), i.e. upper abdominal discomfort or pain that in the opinion of the Investigator is likely to be due to IBS or fulfilling two or more of the criteria: relieved by defecation, associated with change in frequency of stools, associated with change in form (appearance) of stools.
• Inflammatory bowel disease.
• Zollinger-Ellison syndrome.
• Gastric ulcer, duodenal ulcer or duodenal erosions within the last 3 months

Subjects with i) history of significant or ii) current significant or unstable:

• Cardiovascular diseases or cardiac chest pain.
• Cerebrovascular diseases, such as cerebral ischemia, infarction, haemorrhage, or embolus.
• Diabetes mellitus. Stable diabetes controlled on diet, oral agents or insulin is acceptable.
• Pulmonary, renal, pancreatic or liver diseases or any other serious disease as judged by the investigator to interfere with the evaluation of the current study.
• Malignant disease (except for minor superficial skin disease).
• Generalised bleeding disorders.