The authors analysed 45 ultrasonography (US) studies of 43 infants with surgically confirmed hypertrophic pyloric stenosis (HPS) to ascertain whether the current US criteria for pyloric stenosis are applicable to all infants, including those 30 days of age and younger. Most articles in the radiologic literature cite a muscle thickness of 4 mm or greater and a pyloric canal length of 17 mm or greater as diagnostic of HPS. However, the results of this study suggest that in infants 30 days of age or younger the muscle thickness considered diagnostic for HPS be 3 mm or greater.