Introduction: The aim of the study is to evaluate the differences in course of gastroesophageal reflux disease (GERD) depends on co-existing obesity, diabetes and thyroid diseases.Material and methods: 258 patients with GERD were included. In the first stage of the study, the impact of BMI (body-mass index) on the course of GERD was analyzed. Then, the symptoms and gastroscopy results were compared depending on the coexistence of diabetes. The third stage involved patients with GERD and concomitant thyroid diseases. Correlations between anthropometric and biochemical parameters and severity of reflux symptoms and endoscopic image were assessed.Results and conclusions: (1) The results present differences in the clinical course and endoscopic changes in GERD depending on coexisting obesity, diabetes and thyroid diseases. (2) Overweight predispose to esophageal inflammation and increase the frequency of heartburn and sleep problems in GERD patients. (3) Patients with diabetes and GERD are more likely to complain of post-prandial heartburn and have a higher percentage of inflammatory changes in the esophagus. (4) Patients with GERD and thyroid diseases complained more often of burning in the throat and less commonly presented with inflammatory changes in the esophagus. (5) Overweight and obesity in patients with a thyroid disease are not the principal risk factors favoring the development of reflux symptoms.