Infantile hypertrophic pyloric stenosis(IHPS) is common in full-term babies, and relatively rare in prematures. The diagnosis of IHPS in premature infants may be obscured because of the lack of classic symptoms and signs and the absence of the standard criteria for ultrasonic diagnosis. The purpose of this study is to discover the clinical differences between premature and full-term infants with pyloric stenosis, and determine the appropriate diagnostic methods for early diagnosis in premature infants. The clinical records of 52 IHPS patients who had been operated upon from October, 1994 to April, 1997 were reviewed. The incidence of IHPS in premature infants was 25 %. The onset of symptom was 4.7 weeks of age in premature, and 2.9 weeks in full-term babies. Diagnosis was established by typical symptoms. signs. and diagnostic imaging studies. In two premature infants, diagnosis was confirmed by upper gastrointestinal(GI) series, because ultrasonography did not meet the diagnostic criteria. Two premature infants initially diagnosed as gastroesophageal reflux by esophagography. were found to have IHPS by upper GI series. For the diagnosis of IHPS, a new set of criteria for premature babies has to be developed.