The objective of this study was to determine the incidence rate, outcomes, and risk factors of target vessel-related endoleaks after fenestrated-branched endovascular aortic repair (F-BEVAR) for pararenal aneurysms or thoracoabdominal aortic aneurysms (TAAAs). We reviewed consecutive patients treated by F-BEVAR between 2007 and 2017. Target vessel endoleaks were identified by computed tomography angiography (CTA). Follow-up included CTA and duplex ultrasound before discharge, at 2 months, at 6 months, and annually thereafter. Primary endoleaks were detected by predismissal CTA; secondary endoleaks were absent on the first CTA and were identified during follow-up. End points were spontaneous resolution of primary endoleaks, secondary interventions, and aneurysm rupture. Multivariable analyses were performed for risk factors of target vessel endoleaks and predictors of spontaneous resolution. A total of 382 patients (mean age, 75 ± 8 years; 75% male) underwent F-BEVAR for 195 pararenal aneurysms and 187 TAAAs with 1204 renal-mesenteric arteries targeted by 981 fenestrations and 223 directional branches. Fifty-two target vessel endoleaks were identified in 41 patients; 41 were type IIIC (interattachment), 10 were type IC (distal bridging stent sealing zone), and 1 was type IIIB (bridging stent fabric tear). Thirty-three patients (9%) had primary target vessel endoleaks in 41 target vessels (3%). Eight patients (2%) developed 11 secondary target vessel endoleaks. Directional branches were more prone to primary endoleaks (13/223 [6%]) in comparison to fenestrations (28/981 [3%]; P = .03). However, branch endoleaks resolved more often spontaneously (11/14 [79%]) compared with fenestration endoleaks (14/38 [37%]; P = .008). Other risk factors for target vessel endoleaks included inner aortic diameter ≥30 mm at the target vessel origin, four or more targeted vessels, TAAA (for primary endoleaks), and physician-modified endograft (for secondary endoleaks). Four patients with primary endoleaks underwent successful reinterv...