Gastroesophageal reflux is a backflow (reflux or regurgitation) of food and stomach acid into the esophagus, which is the tube that connects the mouth to the stomach. Babies with reflux may lose weight from vomiting or have difficulty eating.

Most infants spit up some of their food, and some healthy babies may spit up frequently. Some babies may spit up because of gastroesophageal reflux. If the muscle (the lower esophageal sphincter) that controls the opening and closing of the valve between the esophagus and the stomach is weak, the backflow of food and stomach acid into the esophagus can irritate the lining of the esophagus, causing burning pain.

Babies who have gastroesophageal reflux may cry because of the burning pain. Reflux in infants can be a further problem if the stomach contents back up into the baby's mouth and are then breathed (aspirated) into the lungs.