The diagnosis is established by the characteristic history of projectile vomiting and the physical finding of a pyloric mass or “olive” on abdominal examination. This may be confirmed by an upper gastrointestinal series but more frequently by ultrasound. The correction of dehydration and acid–base imbalance by adequate parenteral fluid therapy is as important as surgical skill in lowering the mortality rate. Although prolonged gastric intubation is to be avoided, 6 to 12 hours of preparation with intravenous hydration plus suction may be necessary to restore the baby to good physiologic condition. Oral feedings are discontinued as soon as the diagnosis is made, and an intravenous infusion is started in a scalp vein. Then 10 mL/kg of 5% glucose in normal saline is administered rapidly. This is followed by a solution of one part 5% dextrose in normal saline to one part 5% dextrose in water (one-half normal saline with 5% D/W) given at the rate of 150 mL/kg per 24 hours. The baby should be reevaluated every 8 hours with respect to state of hydration, weight, and evidence of edema. Ordinarily, this solution is continued for 8 to 16 hours. After adequate urinary output is established, potassium should be added to the intravenous solutions. In the baby who is moderately or severely dehydrated, it is wise to determine the serum electrolyte values before initiating replacement therapy and to check the values in 8 to 12 hours.