Purpose: To investigate the relation between endoleaks and intraaneurysmal pressure (IAP) and the effect of coil embolization in the management of endoleaks. Methods: The infrarenal aorta of a dog (n = 15) was replaced by a polytetrafluoroethylene aneurysm containing a pressure transducer. Group I (n = 4) had untreated aneurysms. Group II (n = 4) had endovascularly excluded aneurysms without an endoleak. Group III (n = 7) had aneurysms excluded by means of grafts with a defect that represented the source of an endoleak. After 4 weeks of follow-up study, the endoleaks in group III dogs were subjected to coil embolization. Systolic IAP was measured daily and expressed as a ratio of systolic blood pressure obtained from a forelimb cuff. Arteriography, duplex ultrasonography, and spiral contrast computed tomography were performed to evaluate endoleaks. Results: In group I, the IAP remained close to systolic blood pressure (ratio of 0.96 ± 0.06), whereas in group II the IAP ratio showed a decline to 0.34 ± 0.16 (p = 0.0009 group I versus II). After an initial decrease, the IAP ratio in group III stabilized at 0.75 ± 0.18 (p = 0.003, group II versus III). Aneurysms with an endoleak remained pulsatile with a pulse pressure of 30 ± 16 mm Hg, which was less than that of untreated aneurysms (62 ± 15 mm Hg; p < 0.001 group I versus III). Arteriography and computed tomography revealed 'sealing' of endoleaks after coil embolization, but IAP ratio did not decrease (0.76 ± 0.14) after coil embolization. Conclusions: Incomplete endovascular aneurysm exclusion caused by an endoleak fails to reduce IAP ration and may subject the aneurysm to a continued risk for rupture. Although coil embolization resulted in angiographic and computed tomographic sealing, it failed to reduce IAP ratio.