The length, diameter and muscle thickness of the pylorus were measured by ultrasonograms in 15 infants with infantile hypertrophic pyloric stenosis(IHPS). These measurements were compared to assemble measurements of infants who came in for the routine vaccination and underwent ultrasonogram. This study also studied by ultrasound the changes in the pylorus of patients who underwent pyloromyotomy 4 weeks and 8 weeks postoperatively. According to $Carver^5$, 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 length, diameter, thickness and pyloric muscle volume were not normalized at 4 and 8 weeks after pyloromyotomy. However, pyloric muscle index was normalized at 4 weeks postoperatively perhaps as the result of rapid weight gain after pyloromyotomy.