Surgical materials from 20 patients with gastric carcinoma and ten with colonic carcinoma, and autopsy materials from ten patients with pancreatic carcinoma were submitted to the morphometric study of host arteries remaining in and around the tumors, to analyze whether and to what degree the smooth muscles of their media were lost. An explanation would be made of the inertia of tumor-supplying vessels based on their retarded response to vasoactive drugs. It was noted that as arteries advance from a distance toward the inside of a tumor, their media loses smooth muscles gradually, and become amuscular tubes incapable of blood flow regulation. This appears responsible for the abnormal hemodynamics of tumor tissues. Host arteries embedded in cancer were often deformed to aneurysmal or crumpled shapes; this, which also results from relaxation of atonic vessels, was considered to be the pathologic background for the unusual vascular images characteristic of cancer angiograms.