Idiopathic hypertrophic pyloric stenosis(IHPS) is one of the most common causes of persistent nonbilous vomiting in early infancy. The authors retrospectively studied 123 patients with nonbilous vomiting and analyzed the ultrasonographic findings of 51 cases of surgically proven IHPS. No false negative or false positive cases by ultrasonography were confirmed by follow-up clinical observation. US was performed with real-time scanners equipped with a 5MHz transducer(Acuson 128) and a 10MHz transducer(Spectra, Diasonic). All symptomatic infants were first screened in a supine position to determine the amount of gastiric retention. The pylorus, if necessary, the infants were fed E-solution(60-100cc) and then examined. The average sonographic measurements of pyloric muscle thickness(MT), pyloric diameter(PD), and pyloric canal length(PL) were 5.46mm(range 3.6-7.9mm), 15.4mm(range 12.0-18.3mm), and 22.58mm(range 18.0-29.9mm). The ratio of pyloric muscle thickness to the pylorus(MT/PD) was 0.36(range 0.22-0.55). With these measurements, the authors considered the hypertrophic pyloric muscle and the elongated pylorus as a cylinder, and so the pyloric volume was calculated. The pyloric volume(PV), which was equated to 1/4 PD2x PL, was 4.24mL(range 2.62-6.36mL). It was concluded that high-resolution, real-time sonography is a simple and accurate method for the diagnosis of IHPS and should be the initial imaging modality.