In this condition, the pylorus, the passage that leads from the stomach into the small intestine, is narrow because of a thickening of the muscle. The cause is unknown, and it is more common in boys than in girls.

Symptoms usually first appear at two to four weeks, though they can appear earlier or later. The stomach contracts powerfully in an attempt to force a buildup of food through the narrow pylorus. This is impossible, however, and the contents of the stomach are vomited up so violently that they may be propelled up to 3 feet (1 meter) – projectile vomiting. The baby may also suffer constipation and dehydration. Ultrasound and a barium swallow may be used to confirm the diagnosis. A simple operation to widen the pylorus can be performed, curing the condition completely.