The pyloric length, diameter and muscle thickness were measured by ultrasonograms of 15 infants with infantile hypertrophic pyloric stenosis (IHPS) and were compared with the infants who came in for the routine vaccination randomly. This study analyzed the changes in pylorus for those who received pyloromyotomy using the ultrasonogram at 4 weeks and 8 weeks postoperatively, such as the time needed to become normalized, and compare with the size of the hypertrophic pylorus before the pyloromyotomy. According to Carver⁵, the pyloric muscle volume (PMV) and pyloric muscle index (PMI) were calculated in each case. The pyloric muscle volume, PMI and the thickness of pyloric muscle proved to be a more reliable guide to diagnose IHPS than length and diameter of pylorus. The pyloric muscle after pyloromyotomy was measured by ultrasound at 4 weeks and 8 weeks postoperatively. The pyloric muscle length, diameter, thickness and pyloric muscle volume were not normalized at 4 and 8 weeks postoperatively. However, pyloric muscle index was normalized at 4 weeks postoperatively which was probably due to rapid weight gain after pyloromyotomy.