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Hypertrophic pyloric stenosis is a narrowing of the pyloric channel secondary to muscle thickening to the point where emptying of the stomach is prevented. Pyloric stenosis is fairly common as it affects about 3 out of 1,000 babies in the United States. Pyloric stenosis is about four times more likely to occur in firstborn male infants. Pyloric stenosis occurs more commonly in Caucasian infants than in babies of other ethnic backgrounds, and affected infants are more likely to have blood type B or O. Symptoms include progressive, non-bilious projectile vomiting, decreased stooling and no weight gain. Sonography is the imaging modality of choice as the muscles of the pylorus are seen in both the transverse and longitudinal planes, with the additional benefit of realtime visualization of gastric peristalsis. The threshold value of the thickness in the transverse plane from the mucosal margin to the exterior confines of the pyloric musculature is 3mm and the upper limit of normal for channel length is roughly 14 mm. Surgical treatment with pyloroplasty is commonly performed in the United States while medical management is often utilized in Europe.