The incidence of failure of knitted Dacron arterial prostheses is thought to be significantly greater than that reported in the literature. Five cases of immediate and late defects in grafts of various manufacture are reported and a sixth case is discussed. The development of increased porosity to achieve more complete graft healing is thought to play a role in both early and late graft failure. Although the precise etiologic mechanisms are yet to be determined, the Dacron fibers were noted to become separated with subsequent widening of the graft interstices with resulting hemorrhage through the "intact" prosthetic material. Related theories of explanation are reviewed, and guidelines for study of these defective prostheses are suggested. It is essential that this complication of arterial grafting be recognized and that pertinent clinical experiences be documented in the surgical literature.