Gastroesophageal reflux disease (GERD) is a common cause of chronic cough, heartburn, epigastric or retrosternal discomfort, chest pain and abdominal pain or esophagitis. Our patients with OSAS seldom manifest GERD symptoms. We suspected that obesity and high pressure in abdominal cavity may induce acid gastroesophageal reflux in these patients. The aim of the study was to test the hypothesis that obesity, cigarettes smoking or ventilatory and gas exchange abnormalities provoke GERD. We studied 21 consecutive patients with severe OSAS (mean AHI 44.9+/-23.8) before CPAP treatment, all without GERD clinical symptoms. Standard polysomnography, gastroscopy and 24-h oesophageal pH monitoring was performed. There were 6 females, 15 males, mean age 57+/-9 years, mean BMI 38+/-6 kg/m2. All patients presented with normal spirometric and gas exchange values (mean VC 3.64+/-1.23 1, 90% of normal, mean FEV1 2.61+/-0.95 1, 83% of normal, mean FEV1%VC 72%, mean PaO2 68.1+/-7.7 mmHg, mean PaCO2 40.8+/-5.8 mmHg, mean pH 7.42+/-0.02). GERD was diagnosed in 14 patients. Patients with GERD were younger, more often were cigarettes smokers (5/14). We did not fi nd statistically significant differences between severity of OSAS, BMI, ventilatory or gas exchange parameters and GERD.