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The objective of this study was to delineate the specific types of waveforms that exist in type II endoleaks (T2ELs) and their effect on aneurysm sac size.Patients who underwent an endovascular aneurysm repair and were diagnosed with a T2EL were included in the study. The flow velocity characteristics of the T2ELs were evaluated in detail with duplex ultrasound. Four different flow patterns were identified: high resistance, low flow; low resistance, low flow; low resistance, high flow; and to-fro flow. The type and number of vessels involved, time at detection, evolution, and need for treatment were recorded. The aneurysm sac diameter was monitored with duplex ultrasound. A computed tomography scan was always performed at baseline within 1 month of the procedure and repeated only when symptoms developed or there were changes in the ultrasound examination findings, such as sac enlargement.Of 382 patients who underwent endovascular aneurysm repair in our institution, 56 (14.65%) were found to have a T2EL. There were 52 male and four female patients with a mean age of 74 years (61–86 years). The T2EL was diagnosed within the first month in 32 patients; 9 patients were diagnosed at 3 months, 5 patients at 6 months, 3 patients at 9 months, and 7 patients at 1 year or later. There were 43 patients who had a T2EL involving one vessel; two vessels were involved in 11 patients and three vessels were involved in two patients. During follow-up, several changes were observed for the different types of T2EL. High-resistance, low-flow endoleak was detected in 14 patients; of those, 13 were occluded and 1 converted to high flow. Low-resistance, low-flow endoleak was detected in seven patients; of those, 5 were occluded, 1 remained stable, and 1 converted to high flow with sac enlargement requiring treatment. Low-resistance, high-flow endoleak was found in 13 patients; of those, 8 were occluded, 3 remained stable, and 2 had sac enlargement requiring treatment; 1 patient presented with rupture. Finally, to-fro flow was identified in the majority of the patients (22); of those, 14 occluded, 3 remained stable, and 5 had sac enlargement requiring treatment; 2 patients presented with rupture. No deaths due to T2EL were encountered.Most of the T2ELs resulted in spontaneous occlusion and were not associated with sac enlargement. A low-resistance, high-flow or to-fro flow T2EL has higher chances of sac enlargement, rupture, and requiring reintervention.