Target artery (TA) instability has been the most frequent indication for secondary intervention after fenestrated and branched endovascular aortic repair (FB-EVAR) of pararenal and thoracoabdominal aortic aneurysms (TAAAs). The aim of the present study was to evaluate the effect of the gap distance between the endograft reinforced fenestration and TA origin at the aortic wall (fenestration gap [FG]) on target-related outcomes after FB-EVAR. The clinical data and imaging studies of 430 patients enrolled in a prospective, nonrandomized study to evaluate FB-EVAR using manufactured stent grafts were reviewed. Of the 430 patients, 340 (79%) had had more than one vessel incorporated by fenestration. The FG was retrospectively measured on postoperative imaging studies and classified into three groups: no gap (FG, 0 mm), FG 1 to 4 mm, and FG ≥5 mm. The primary outcome was freedom from TA instability. The secondary end points included TA-related endoleak, TA secondary intervention, and TA patency. A total of 1558 renal-mesenteric TAs were incorporated by 1104 reinforced fenestrations and 454 directional branches (DBs), with a mean of 3.9 ± 0.5 vessels per patient. The mean FG was 2.8 ± 4.5 mm, with an FG of 0 mm for 646 TAs, 1 to 4 mm for 209 TAs, and ≥5 mm for 249 TAs. An FG of ≥5 mm was associated with significantly lower ( P < .001) freedom from TA instability, type Ic or IIIc endoleak, and secondary interventions at 5 years. Compared with DBs, fenestrations with an FG of ≥5 mm had similar primary patency and freedom from TA instability but significantly lower freedom from type Ic or IIIc endoleak (91% ± 2% vs 95% ± 1%; log rank, P = .02) and secondary interventions (87% ± 3% vs 93% ± 2%; log-rank, P = .02) at 5 years. The independent predictors of TA instability included postdissection TAAAs (hazard ratio, 2.5; 95% confidence interval, 1.2-5.4) and FG ≥5 mm (hazard ratio, 1.6; 95% confidence interval, 1.2-1.8). TAs incorporated by reinforced fenestrations had higher primary (99% ± 0.8% vs 97% ± 1.0%; P = .039) and sec...