Background: Impaired gastric accommodation of the proximal stomach is one of the major pathophysiological mechanisms in functional dyspepsia (FD). However, no useful method exists for the clinical evaluation of this phenomenon. Aim: The aim of the present study was to establish a simple and non-invasive method for evaluating the accommodation reflex of the proximal stomach. Methods: Nine healthy subjects received up to 1,700 mL water (stepwise administration in 100-mL increments) using a nasogastric tube while they were in a supine position. To assess the meal-induced gastric accommodation reflex, we measured the cross-sectional area of the proximal stomach via ultrasonography (US) at 3-min intervals after administration of water. We also measured the pressure of the water column using the same tube. Then, we administrated up to 400 mL of water in 100-mL increments and measured the area of the proximal stomach in 44 FD patients with early satiation (the measurements were performed at intervals of 3-min), and we compared the results with those for 44 healthy subjects. Results: The incremental changes in the area of the proximal stomach corresponded well with the amount of water administered. The area of the proximal stomach increased, but the antral area and the intragastric pressure remained relatively stable. After administration of more than 100 ml water, the area of the proximal stomach in healthy subjects was significantly greater than that in FD patients. Conclusion: US can be used to assess the isotonic expansion of the proximal stomach. We were able to distinguish FD patients with impaired accommodation reflex from healthy individuals by using this simple and easy method.