Background　and　objectives:　Initiation,　growth,　and　rupture　of　intracranial　aneurysms　are　believed　to　be　closely　related　to　their　local　haemodynamic　environment.　While　haemodynamics　can　be　characterised　by　use　of　computational　fluid　dynamics　(CFD),　its　reliability　depends　heavily　upon　accurate　assumption　of　the　boundary　conditions.　Herein,　we　compared　the　simulated　aneurysmal　haemodynamics　obtained　by　use　of　generic　boundary　conditions　against　those　obtained　under　flow　conditions　measured　in　vivo　.　Methods:　We　prospectively　recruited　19　patients　with　intracranial　aneurysms　requiring　3-dimensional　rotational　angiography,　during　which　blood　pressure　at　the　internal　carotid　artery　was　probed　by　catheter　and　flowrate　measured　by　a　dedicated　software　tool.　Using　these　flow　conditions　measured　in　vivo　,　we　quantified　the　aneurysmal　haemodynamics　for　each　patient　by　CFD,　and　then　compared　the　results　with　those　derived　from　a　generic　condition　reported　in　the　literature,　in　terms　of　the　time-averaged　wall　shear　stress　(TAWSS),　oscillatory　shear　index　(OSI),　relative　residence　time　(RRT),　and　percentage　of　the　intra-aneurysmal　flow　(PIAF).　In　addition,　the　effects　on　aneurysmal　haemodynamics　of　different　outflow　strategies　(splitting　method　vs　.　Murray＇s　Law)　and　simulation　schemes　(transient　vs　.　steady-state)　relative　to　each　flow　condition　were　also　assessed.　Results:　Differences　in　the　simulated　TAWSS　(　-6.08　+/-　10.64　Pa,　p　=　0.001),　OSI　(0.06　+/-　0.13,　p　=　0.001),　and　PIAF　(　-0.05　+/-　0.20,　p　=　0.012)　between　the　patient-specific　and　generic　boundary　conditions　were　found　to　be　statistically　significant,　in　contrast　to　that　in　the　RRT　(49　+/-　307　Pa　-1,　p　=　0.062).　Out-flow　strategies　did　not　yield　statistically　significant　differences　in　any　of　the　investigated　parameters　(all　p　＞　0.05);　rather,　the　resulting　parameters　were　found　to　be　in　good　correlations　(all　r　＞　0.71,　p　＜　0.001).　Difference　between　the　aneurysmal　TAWSS　and　the　WSS　derived　from　cycle-averaged　flowrate　condition　was　found　to　be　minor　(0.66　+/-　1.36　Pa,　p　=　0.0　0　0),　so　was　that　between　PIAFs　obtained　respectively　from　the　transient　and　steady-state　simulations　(0.02　+/-　0.05,　p　=　0.0　0　0).　Conclusions:　Incorporating　into　simulation　the　patient-specific　boundary　conditions　is　critical　for　CFD　to　characterise　aneurysmal　haemodynamics,　while　outflow　strategies　may　not　introduce　significant　uncer-tainties.　Steady-state　simulation　incorporating　the　cycle-averaged　flow　condition　may　produce　unbiased　WSS　and　PIAF　compared　to　the　transient　analysis.　(c)　2022　Elsevier　B.V.　All　rights　reserved.