For patients who have undergone gastrectomy for gastric cancer, a follow-up by upper gastrointestinal endoscopy is required. However, it is sometimes very difficult to observe the remnant stomach due to a significant amount of residual food. We evaluated the reduction of food residue by drinking water as preparation before upper gastrointestinal endoscopy in postpartial gastrectomy patients. We compared two groups. In the water group, patients drank 500 mL water after dinner on the evening before endoscopy, and on the day of endoscopy they drank 350 mL water before the examination. In the control group, patients drank nothing after dinner, and nothing on the day of endoscopy. In the water group, food residue in the remnant stomach was reduced. In patients who underwent proximal gastrectomy or pylorus-preserving gastrectomy, food residue in the remnant stomach was not reduced by drinking water. However, in patients with distal gastrectomy, food residue was reduced by drinking water. Drinking water before upper gastrointestinal endoscopy is safe with no risk of complications, and our results suggested that drinking water effectively reduced food residue in the remnant stomach before endoscopic examination.