BACKGROUND Most frequently, abdominal aortic aneurysm (AAA) ruptures into retroperitoneal space. The rupture of AAA into inferior vena cava is an uncommon event. The incidence of this complication of AAA is 2 to 10%. Surgeons' awareness of this rare entity is the most important factor for the early diagnosis and treatment. In this paper we report two cases of AAA rupture into inferior vena cava. As to our knowledge, in domestic literature such cases have not been previously reported. METHODS Patient 1. A 65-year-old man was admitted to the hospital because of low back pain and haemorrhagic shock. He was anaemic with haemoglobin of 80 g/l, systemic blood pressure was 70 mmHg, pulse rate 100/min, and central venous pressure 12 cm H2O. Pulsatile abdominal mass with continuous bruit and thrill and leg oedema were present. Physical examination revealed global heart failure. The patient was anuric. Because of the critical condition and evident clinical signs of ruptured AAA, the patient was operated on immediately without any other diagnostic procedure. Transperitoneal approach was used. Intraoperative findings were consistent with the rupture of the frontal aneurysmal wall into retroperitoneal space, with large retroperitoneal haematoma and aorto-caval (AC) fistula on the posterior aneurysmal wall, large 2 cm in diameter. Using digital compression for venous bleeding control, the fistula was closed with interrupted polypropylene 2-0 sutures with patches. After closure of the fistula, the urine flow resumed. Then, the aneurysm was replaced with bifurcated Dacron graft. The postoperative recovery was successful. The patient has a 13-year follow-up, without any sign of cardiac or renal failure as well as arterio-venous insufficiency of legs. Patient 2. A 62-year-old man was admitted to the Zemun Clinical Hospital Cenre because of suddenly occurred tachycardia, dyspnea and low back pain. Abdominal ultrasound examination revealed the existence of a possible fistula between the abdominal aorta and inferior vena cava. The patient was immediately transported to our institute. At admission, he was anaemic (haemoglobin was 85 g/l), with systolic blood pressure of 100 mmHg, pulse rate of 100/min and central venous pressure of 20 cm H2O. Also, he had pulsatile abdominal mass with continuous bruit and thrill, as well as legs and scrotal oedema. He was oliguric and haematuric. Translumbar aortography showed AAA with AC fistula (Figure). Transperitoneal approach was used for the operation. Intraoperatively, a small retroperitoneal haematoma without retroperitoneal rupture was found. After aneurysmal opening, a massive venous bleeding started, followed with cardiac arrest. The bleeding was controlled using digital compression and cardiopulmonary resuscitation was successful. AC fistula, large 3 cm in diameter, was on the posterior aneurysmal wall, and it connected the inferior vena cava and the left common iliac vein with AAA. The fistula was closed with interrupted polypropylene 2-0 sutures with patches. The aneurysm was replaced with impregnated tubular Dacron graft 16 mm. The postoperative recovery was successful. The patient was followed-up for 2.5 years, and there were no signs of cardiac or renal failure and arterio-venous insufficiency of legs. CONCLUSIONS AC fistula as a complication of ruptured AAA was reported for the first time by Syme in 1831. The first attempt to repair this lesion was done by Lehman in 1935, but it was unsuccessful. In 1954, the first successful repair was performed by Cooley. According to Matsubara, by the end of 1989, 250 cases of this lesion were reported in England, German and French literature, and only 25 in Japanese. In 1991, Brewster et al. reported 14 new cases, while Italian authors reported 36 new cases in 1994. Retroperitoneal and intraperitoneal ruptures of AAA have different clinical presentation comparing with the rupture of AAA into inferior vena cava. (ABSTRACT TRUNCATED)