In 14 human autopsy specimens, obtained within two hours of death, the contribution of external ligamentous attachments to competence against reflux at the ileocecal junction (ICJ) was evaluated. The ascending colon was filled with saline by retrograde flow, and pressures at which coloileal reflux occurred were recorded. Twelve of 14 ICJ's were competent to pressures of up to 80 mm Hg; two incompetent ICJ's refluxed fluid into the ileum at intracecal pressures of approximately 40 mm Hg. Competent specimens were then restudied. Removal of mucosa at the ileocecal junction (N = 6), or a strip of circular muscle (N = 6), did not impair competence to pressures above 40 mm Hg. However, division of fibrous tissues which helped maintain an angulation between the ileum and cecum (superior and inferior ileocecal ligaments) rendered the junction incompetent in all specimens. In four samples tested, surgical reconstruction of the ileocecal angle restored competence. Comparable observations were made in three anesthetized dogs in vivo. These findings suggest that mechanical factors, maintained by the external anatomy, contribute to competence at the ICJ.