Infantile hypertrophic pyloric stenosis (IHPS) is characterized by hypertrophy of the pyloric muscle, causing pyloric-channel narrowing and elongation. The rigidity of the muscle is increased, which is characterized as an "olive." Elastin is an insoluble protein that forms the major structural component of the elastic fibers and maintains the tensile strength of the tissues. To understand the possible histologic and molecular basis of the elasticity of the hypertrophic muscle in IHPS, we determined the distribution of the elastic fibers and elastin expression using Victoria blue van Gieson (VVG) staining and immunohistochemistry. In IHPS, the number of elastic fibers in the connective tissue was significantly increased in the thickened connective-tissue septa (CTS) compared with normal control specimens. In normal pyloric muscle, weak to moderate elastin immunoreactivity was observed in the CTS while no immunoreactivity was observed among the muscle fibers. In IHPS, strong immunoreactivity of elastin was observed in the CTS and moderate immunoreactivity among the hypertrophic smooth-muscle fibers. Our findings suggest that the increase on elastic fibers and elastin expression in the pyloric muscle in IHPS may play an important role in the development of pyloric-muscle rigidity, causing pyloric-canal obstruction.