The effects of prostaglandin (PG) F2 alpha in 16 patients with vascular malignant intestinal tumors were analyzed by angiography. It was found that PGF2 alpha reduced tumor vascular flow selectively in all but one patient, a rectal carcinoma case. Among the remaining group, a case of intestinal choriocarcinoma complicated by massive gastrointestinal hemorrhage was successfully controlled with intraarterial infusion of PGF2 alpha into the superior mesenteric artery. Owing to the reduced blood flow in tumors, PGF2 alpha is expected to be used extensively as a vasoconstrictor to control bleeding from tumors of the alimentary tract.