Most infantile hypertrophic pyloric stenosis (IHPS) cases are diagnosed between 3 and 12 weeks after birth. Few data exist regarding Asian infants with IHPS who are younger than 3 weeks or are preterm. The goal of this study is to identify unusual clinical manifestations, clinical course, duration of hospital stay, and complications of Asian infants with IHPS who are preterm or younger than 3 weeks of age. From 1991 to 2004, all IHPS patients admitted to three tertiary centers in southern Taiwan were enrolled. The clinical manifestations, duration of hospital stay and complications were further compared between the IHPS patients diagnosed before and after 3 weeks; preterm and term infants. A total of 214 patients were enrolled into the study; the mean age of diagnosis was 40 days of age; the average duration of hospital stay was 6.27 days. Eighteen (8.41%) patients were diagnosed before 3 weeks of age. A significantly shorter timeframe of diagnosis, a higher rate of jaundice, a lower daily body weight gain and longer duration of hospital stay were noted in the IHPS group prior to 3 weeks compared with those in IHPS group after 3 weeks. Eighteen were preterm infants. A significantly older age of symptom onset, a lower body weight at admission, more cases diagnosed by barium meal study and higher postoperative complication rates were noted in the preterm group versus full-term infants with IHPS. The IHPS cases diagnosed before 3 weeks of age had longer duration of hospital stay. Preterm infants with IHPS had more postoperative complications. [ABSTRACT FROM AUTHOR]