Measuring antral cross-sectional area by ultrasonography can be an ideal way to evaluate intragastric milk volume in infants. Technical details, however, remain to be established before its clinical application. We investigated the effects of posture and ultrasonographic plane on the correlation between milk volume and antral cross-sectional area. After gastric aspiration through a nasogastric tube, healthy newborns were fed 0, 10, 20, and 40 ml of milk cumulatively, and antral cross-sectional area was measured in either upright, sitting, or right lateral position. To determine the best sonographic plane, subjects were put in the right lateral position and antral cross-sectional area was measured in the plane of the aorta and either the superior mesenteric artery, the superior mesenteric vein, the midline of the abdominal surface, 1 cm right of midline, or 2 cm right of midline. The results showed that antral cross-sectional area reflects intragastric milk volume most accurately, with minimal gas interference when measured in the right lateral position. The area correlates well with milk volume in the plane of the aorta and either the superior mesenteric artery, the superior mesenteric vein, or the midline. Next, we studied the effect of intragastric gas on the antral cross-sectional area in subjects who were given 40 ml of milk followed by an injection of air. More than 20 ml of intragastric gas increases antral cross-sectional area significantly. Ultrasonographic evaluation of intragastric volume requires attention to the above factors.