Inferior vena cavography is frequently performed in conjunction with excretory urography and lymphangiography for the evaluation of intra-abdominal or retroperitoneal masses. Collateral circulation occurs in intrinsic obstruction with thrombosis or extrinsic compression of the inferior vena cava. Although frequently mentioned, the portal venous system as a major collateral in lower inferior vena caval obstruction is not commonly visualized at cavography. The purpose of this communication is to report two cases in which the portal venous system was visualized as a principal collateral route. Case I (T. S.): This patient was a 60-year-old white female, gravida V, Para 2, who had Stage IV epidermoid carcinoma of the cervix with urinary bladder involvement in December 1959. She was treated with intracavitary radium (3,000 mg hr.) followed by external radiation of 3,994 R/t/4 wk. The patient returned in February 1961 with recurrent tumor which was treated by an anterior pelvic exenteration and bilateral ureterosigmoidostomy. The patient did well until April 1962 when she was readmitted with a four-month history of swelling of both lower extremities and pubic region, pain in the right hip and thigh, intermittent low backache, and recurrent chills and fever. On admission, pelvic venography showed obstruction of both common iliac veins and a pelvic mass of a conglomeration of veins which subsequently opacified the inferior mesenteric vein and the portal venous circulation (Fig. 1). The patient died on Sept. 12, 1962, from generalized carcinomatosis. Case II (A.M.): This 24-year-old gravida IV, Para 4, Negress had profuse vaginal bleeding in December 1964. The patient had a Stage I epidermoid carcinoma of the cervix. She was treated with intracavitary radium (3,500 mg hr. in April 1965 and an additional 3,500 mg hr. in May 1965). External radiation (3,500 R/t/4 wk.) was administered via split ports to the pelvis. In November 1965 the patient complained of frequency of bowel movements and blood-tinged stool, urgency, and dysuria. Barium-enema examination on December 1965 revealed a rectosigmoid stricture. A transverse colostomy was performed. Exploration of the abdomen at this time did not disclose any tumor. In December 1966 right lower extremity edema and pain developed, radiating down the right leg. Biopsy of the uterine cervix at this time showed recurrent epidermoid carcinoma. Pelvic venography on Dec. 15, 1966, demonstrated obstruction of both common iliac veins. A mass of venous plexus was seen in the pelvis with subsequent opacification of the inferior mesenteric vein and the portal venous system (Fig. 2). This phenomenon was observed when 60 cc of methylglucamine diatrizoate was rapidly injected into the right femoral vein. The inferior mesenteric vein was not opacified when 40 cc of contrast material was manually injected into the left femoral vein. In both examinations, the vertebral plexus of veins was well demonstrated.