• Hypertrophic pyloric stenosis (HPS) typically occurs in newborns between weeks 2 and 8 of life. Incidence is 1 in 300–900 live births. • Approximately four times more common in boys than in girls. • Hypertrophy of the pyloric muscle results in constriction and obstruction of the gastric outlet. • The classic presentation of HPS is non-bilious projectile vomiting in a 2–8-week-old, full-term neonate. At first, emesis is infrequent and then progresses to projectile vomiting with every feed—classically a non-bloody, non-bilious emesis although “coffee-ground” emesis can occur with gastritis/esophagitis. • Early in course the baby is well appearing (happy, hungry vomiter); later in course as dehydration develops, neonate can be gaunt and somnolent. • Abnormality is not congenital; risk increases with positive family history. • Ultrasound is the gold standard for diagnosis: pyloric muscle thickness of 4 mm (3.5 in premature infants) and a length of 16 mm or greater.  – Upper gastrointestinal imaging can also be utilized and can see an elongated pyloric channel; barium should be seen exiting the stomach to exclude pyloric web or atresia. • Differential: GERD, gastroenteritis, increased intracranial pressure, malrotation, gastric tumors, foregut webs, or duplications cysts.