WHAT THIS PAPER ADDS This is the largest study to date on abdominal compartment syndrome (ACS) after abdominal aortic aneurysm (AAA) repair, including intact and ruptured AAAs. ACS subgroup outcomes and the impact of duration of intraabdominal hypertension (IAH) were investigated. Outcome was poor regardless of whether ACS was associated with post-operative bleeding, bowel ischaemia, or oedema, and regardless of timing of decompression. The duration of IAH before treatment predicted the need for renal replacement therapy. This emphasises the need to focus on prevention, through careful monitoring of intra-abdominal pressure, strategies for pre-emptive treatment of IAH, and swift treatment when ACS develops. Objectives: Abdominal compartment syndrome (ACS) is a serious complication after abdominal aortic aneurysm (AAA) repair. The aim was to investigate outcome among subgroups and factors associated with outcome, with emphasis on the duration of intra-abdominal hypertension before treatment. Methods: Since 2008, ACS and decompressive laparotomy (DL) after AAA repair are registered prospectively in the Swedish vascular registry (Swedvasc). Registry data and case records were reviewed. Subgroups were defined by main pathophysiological finding at DL, timing of DL after AAA repair, and treatment modality. Results: During 2008e2015, 120 of 8765 patients undergoing surgery for infrarenal AAA developed postoperative ACS (1.4%). Eighty-three followed ruptured AAA (rAAA); 45 open surgical repairs (OSR) and 38 endovascular (EVAR), and thirty-seven after intact AAA (iAAA); 30 OSR and seven EVAR. The main pathophysiological findings at DL were bowel ischaemia in 27 (23.3%), post-operative bleeding in 34 (29.3%), and general oedema in 55 (47.4%). DL was performed <24 hours after AAA repair in 56 (48.7%), 24e48 hours in 30 (26.1%), and >48 hours in 29 patients (25.2%). The overall 90 day mortality was 50.0%, neither different depending on main pathophysiological finding, nor on the timing of DL. In multivariable regression analysis, age was a predictor of mortality (p ¼ .017), while duration of intra-abdominal hypertension (IAH) prior to DL predicted the need for renal replacement therapy (RRT) (p ¼ .033). DL was performed earlier after EVAR compared with OSR in rAAA (p < .001). Conclusions: Mortality in ACS was high, irrespective of the main pathophysiological finding and timing of DL. The duration of IAH prior to DL predicted the need for RRT. DL was performed earlier after EVAR than after OSR for rAAA, underlining the importance of monitoring IAP after EVAR for rAAA.