INTRODUCTION: Uncontrolled results suggest that diaphragmatic breathing (DB) is effective in gastroesophageal reflux disease (GERD) but the mechanism of action and rigor of proof is lacking. This study aimed to determine the effects of DB on reflux, lower esophageal sphincter (LES), and gastric pressures in patients with upright GERD and controls. METHODS: Adult patients with pH proven upright GERD were studied. During a high-resolution impedance manometry, study patients received a standardized pH neutral refluxogenic meal followed by LES challenge maneuvers (Valsalva and abdominal hollowing) while randomized to DB or sham. After that, patients underwent 48 hours of pH-impedance monitoring, with 50% randomization to postprandial DB during the second day. RESULTS: On examining 23 patients and 10 controls, postprandial gastric pressure was found to be significantly higher in patients compared with that in controls (12 vs 7 mm Hg, P = 0.018). Valsalva maneuver produced reflux in 65.2% of patients compared with 44.4% of controls (P = 0.035). LES increased during the inspiratory portion of DB (42.2 vs 23.1 mm Hg, P < 0.001) in patients and healthy persons. Postprandial DB reduced the number of postprandial reflux events in patients (0.36 vs 2.60, P < 0.001) and healthy subjects (0.00 vs 1.75, P < 0.001) compared with observation. During 48-hour ambulatory study, DB reduced the reflux episodes on day 2 compared with observation on day 1 in both the patient and control groups (P = 0.049). In patients, comparing DB with sham, total acid exposure on day 2 was not different (10.2 ± 7.9 vs 9.4 ± 6.2, P = 0.804). In patients randomized to DB, esophageal acid exposure in a 2-hour window after the standardized meal on day 1 vs day 2 reduced from 11.8% ±6.4 to 5.2% ± 5.1, P = 0.015. DISCUSSION: In patients with upright GERD, DB reduces the number of postprandial reflux events pressure by increasing the difference between LES and gastric pressure. These data further encourage studying DB as therapy for GERD.