Infants with hypertrophic pyloric stenosis typically present at 2 to 4 weeks of age with nonbilious projectile vomiting. Hypertrophic pyloric stenosis is exceedingly rare in newborn infants and is scarcely reported in literature. Also, the diagnostic criteria for ultrasonographic measurements in newborn infants have yet to be determined. This report is of a newborn infant with hypertrophic pyloric stenosis. The patient presented with high-volume non-bile- stained output from a nasogastric tube and a dilated gastric bubble on abdominal radiograph. Contrast study ruled out intestinal malrotation. Two ultrasound tests showed that the pyloric muscle thickness and pyloric canal length were within normal limits. Subsequent laparotomy showed a thickened pylorus and pyloromyotomy was performed. The patient showed marked improvement in feeding postoperatively. A high index of suspicion is required for newborn infants presenting with gastric outlet obstruction. Ultrasound and contrast studies provide additional information, but definitive diagnosis may only be available intra_operatively.