OBJECTIVE The purposes of this study were to evaluate the accuracy of the ultrasonographic measurements used during diagnosis of hypertrophic pyloric stenosis (HPS), to examine the relationship between the palpability and the morphologic features of the pyloric mass, and to show the morphologic resolution of the pylorus after pyloromyotomy. METHODS One hundred three infants with the diagnosis of HPS by clinical findings and ultrasonographic measurements were treated surgically by Ramstedt pyloromyotomy. The measurement parameters were the muscle thickness, diameter, and channel length of the pylorus. The serial measurements were performed postoperatively for 1 year after surgery. RESULTS The preoperative ultrasonographic measurements of the pylorus were significantly correlated with the actual measurements obtained by surgery (P<.05). The diameter correlated most significantly with the palpability of the pylorus. In contrast to the prompt improvement of clinical symptoms, the muscle thickness did not reach the normal range (approximately 2 mm) until 8 months after surgery. The diameter had not decreased to its normal range (approximately 10 mm) by the end of the 1-year postoperative follow up period. The channel length decreased more rapidly than the other 2 parameters. CONCLUSIONS The preoperative ultrasonographic measurements of the pylorus reliably evaluated the pyloric muscle thickness and diameter in HPS. The actual pyloric diameter correlated with the palpability of the pyloric mass. The morphologic resolution of the pylorus after surgery did not correlate with the prompt improvement of symptoms.