The ileocolonic junctional region exhibits many features considered characteristic of a gastrointestinal sphincter. Thus the junction between large and small bowel is marked in many species by considerable thickening of the circular muscle coat, and physiological studies have revealed properties similar to other sphincteric areas. The region generates a tonic pressure and exhibits responses to distension, nerve stimulation and pharmacological agents clearly different from adjacent ileum and colon. The factors responsible for generating and maintaining tone within this sphincter remain unclear, however. While extrinsic nerves do not appear to play a significant role, importance has been variously attributed to the contributions of intrinsic nerves, myogenic tone and elastic factors. The function of this region in the intact animal has not been extensively investigated or clearly defined. However observations in both animals and man suggest that the ileocolonic sphincter acts in concert with adjacent ileum to store chyme in the distal small bowel and periodically expel it into the colon. Coordinated activity is especially noticeable following food when increased ileal activity combined with relaxation of the sphincter promotes rapid emptying of ileal contents. Clinical and experimental studies indicate that the presence of an intact ileocolonic sphincter may increase survival and reduce morbidity following extensive small bowel resection. By preventing reflux of colonic contents the sphincter may serve to minimize colonization of the small bowel by large bowel bacterial flora. The response of the ileocolonic sphincter to large bowel distension appears highly variable and may depend more on the magnitude of the intracolonic pressure generated rather than on inherited or acquired abnormalities of junctional zone anatomy.