A 10-cm-diameter iliac venous aneurysm presented as an adnexal mass in a pregnant patient with a renal transplant. She also had a femoral-saphenous arteriovenous shunt for facilitation of dialysis prior to the transplant. Diagnosis was made at laparotomy. The aneurysm was thought to have arisen because of elevation of iliac venous pressure due to the fistula, vascular relaxation and pressure of the uterus on the pelvic veins.