We compared sonographic images and measurements of patients diagnosed as having hypertrophic pyloric stenosis and pylorospasm among infants with projectile vomiting. Thirty‐seven patients with hypertrophic pyloric stenosis had an unchanged pyloric length (mean, 22.5 mm) and muscle wall thickness (mean, 5.3 mm). Thirty‐four pylorospasm patients had considerable variability in measurement or image appearance during their studies. Means of their longest or largest measurements were 14.4 mm for pylorus length and 3.8 mm for muscle wall thickness. Among these, 53% had muscle wall thickness of 4 mm or greater and 18% had pyloric length of 18 mm or greater during some portion of their study. We concluded (after clinical follow‐up study confirmed our ultrasonographic diagnoses) that pylorospasm may mimic hypertrophic pyloric stenosis for at least a portion of a sonographic study. Muscle wall thickness or pyloric length measurements may overlap those accepted as positive for hypertrophic pyloric stenosis. Image or measurement variability is an important clue for diagnosing pylorospasm.