Thirteen infants were examined with high-resolution real time ultrasound following Rammstedt pyloromyotomy for hypertrophic pyloric stenosis and 6 patients were followed up to 3 months. Our results were as follows. 1.In the preoperative study, all cases with hypertrophic pyloric stenosis demonstrated the pyloric muscle thickness to be 4mm or greater, the pyloric diameter to be 13mm or greater, the pyloric channel length to be 17mm or greater, the pyloric muscle volume to be 2.25㎤ or greater and the pyloric muscle index to be 0.61 or greater, respectively. 2. Operative measurement of the pyloric muscle thickness was between 2 and 6mm, the pyloric diameter was 14 and 25mm, the pyloric channel length was between 18 and 31mm, the pyloric muscle volume was 1.85 and 8.18㎤, and pyloric muscle index was 0.46 and 1.70, showing good correlation with ultrasonographic measurements. 3.Sequential sonograms showed that it took 3 months for the muscle hypertrophy to resolve in all of the six patients. And in the postoperative evaluation, pyloric muscle thickness and pyloric muscle index were valuable parameters. In conclusion, following longitudinal pyloromyotomy, the hypertrophied pylorus returns to normal by 3 months.