Objective: To assess the incidence rate of oesophageal adenocarcinoma among patients with non-erosive gastro-oesophageal reflux disease compared with the general population. Design: Population based cohort study. Setting: All patients in hospital and specialised outpatient healthcare in Denmark, Finland, and Sweden from 1 January 1987 to 31 December 2019. Participants: 486 556 adults (>18 years) who underwent endoscopy were eligible for inclusion: 285 811 patients were included in the non-erosive gastro-oesophageal reflux disease cohort and 200 745 patients in the validation cohort with erosive gastro-oesophageal reflux disease. Exposures: Non-erosive gastro-oesophageal reflux disease was defined by an absence of oesophagitis and any other oesophageal diagnosis at endoscopy. Erosive gastro-oesophageal reflux disease was examined for comparison reasons and was defined by the presence of oesophagitis at endoscopy. Main outcome measures: The incidence rate of oesophageal adenocarcinoma was assessed for up to 31 years of follow-up. Standardised incidence ratios with 95% confidence intervals were calculated by dividing the observed number of oesophageal adenocarcinomas in each of the gastro-oesophageal reflux disease cohorts by the expected number, derived from the general populations in Denmark, Finland, and Sweden of the corresponding age, sex, and calendar period. Results: Among 285 811 patients with non-erosive gastro-oesophageal reflux disease, 228 developed oesophageal adenocarcinomas during 2 081 051 person-years of follow-up. The incidence rate of oesophageal adenocarcinoma in patients with non-erosive gastro-oesophageal reflux disease was 11.0/100 000 person-years. The incidence was similar to that of the general population (standardised incidence ratio 1.04 (95% confidence interval 0.91 to 1.18)), and did not increase with longer follow-up (1.07 (0.65 to 1.65) for 15-31 years of follow-up). For validity reasons, we also analysed people with erosive oesophagitis at endoscopy (200 745 patients, 1 750 249 person-years, and 542 oesophageal adenocarcinomas, corresponding to an incidence rate of 31.0/100 000 person-years) showing an increased overall standardised incidence ratio of oesophageal adenocarcinoma (2.36 (2.17 to 2.57)), which became more pronounced with longer follow-up. Conclusions: Patients with non-erosive gastro-oesophageal reflux disease seem to have a similar incidence of oesophageal adenocarcinoma as the general population. This finding suggests that endoscopically confirmed non-erosive gastro-oesophageal reflux disease does not require additional endoscopic monitoring for oesophageal adenocarcinoma.