Infantile hypertrophic pyloric stenosis (IHPS) is characterized by hypertrophy of the pyloric muscle. The growth of smooth muscle cells is regulated by several growth factors. Epidermal growth factor (EGF) and heparin-binding EGF-like growth factor are potent mitogens for smooth muscle cells. In the present study, we investigated immunohistochemical localization of EGF and EGF-related peptides and EGF mRNA expression in pyloric smooth muscle cells to determine whether the EGF family is involved in the process of pyloric muscle hypertrophy in IHPS. Pyloric muscle biopsy specimens were obtained at the time of pyloromyotomy from 10 patients with IHPS. Control material included 10 pyloric muscle specimens taken at autopsy from age-matched cases without evidence of gastrointestinal disease. Indirect immunohistochemistry was performed using the avidin-biotin-peroxidase complex method with anti-EGF, anti-EGF receptor, and anti–heparin-binding EGF-like growth factor antibody.In situhybridization was performed using digoxigenin-labeled EGF-specific oligonucleotide probe. The pattern of immunoreactivity in pyloric muscle with EGF, EGF receptor, and heparin-binding EGF-like growth factor was similar in all specimens. There was a marked increase in EGF, EGF receptor, and heparin-binding EGF-like growth factor immunoreactivity and EGF mRNA expression in smooth muscle cells in pyloric circular and longitudinal muscle from patients with IHPS compared with control specimens. These data suggest that the up-regulated local synthesis of EGF and EGF-related peptides in pyloric muscle may play a critical role in the development of pyloric muscle hypertrophy in IHPS.