Validation　studies　are　prerequisites　for　computational　fluid　dynamics　(CFD)　simulations　to　be　accepted　as　part　of　clinical　decision-making.　This　paper　reports　on　the　2011　edition　of　the　Virtual　Intracranial　Stenting　Challenge.　The　challenge　aimed　to　assess　the　reproducibility　with　which　research　groups　can　simulate　the　velocity　field　in　an　intracranial　aneurysm,　both　untreated　and　treated　with　five　different　configurations　of　high-porosity　stents.　Particle　imaging　velocimetry　(PIV)　measurements　were　obtained　to　validate　the　untreated　velocity　field.　Six　participants,　totaling　three　CFD　solvers,　were　provided　with　surface　meshes　of　the　vascular　geometry　and　the　deployed　stent　geometries,　and　flow　rate　boundary　conditions　for　all　inlets　and　outlets.　As　output,　they　were　invited　to　submit　an　abstract　to　the　8th　International　Interdisciplinary　Cerebrovascular　Symposium　2011　(ICS＇11),　outlining　their　methods　and　giving　their　interpretation　of　the　performance　of　each　stent　configuration.　After　the　challenge,　all　CFD　solutions　were　collected　and　analyzed.　To　quantitatively　analyze　the　data,　we　calculated　the　root-mean-square　error　(RMSE)　over　uniformly　distributed　nodes　on　a　plane　slicing　the　main　flow　jet　along　its　axis　and　normalized　it　with　the　maximum　velocity　on　the　slice　of　the　untreated　case　(NRMSE).　Good　agreement　was　found　between　CFD　and　PIV　with　a　NRMSE　of　7.28%.　Excellent　agreement　was　found　between　CFD　solutions,　both　untreated　and　treated.　The　maximum　difference　between　any　two　groups　(along　a　line　perpendicular　to　the　main　flow　jet)　was　4.0　mm/s,　i.e.　4.1%　of　the　maximum　velocity　of　the　untreated　case,　and　the　average　NRMSE　was　0.47%　(range　0.28-1.03%).　In　conclusion,　given　geometry　and　flow　rates,　research　groups　can　accurately　simulate　the　velocity　field　inside　an　intracranial　aneurysm-as　assessed　by　comparison　with　in　vitro　measurements-and　find　excellent　agreement　on　the　hemodynamic　effect　of　different　stent　configurations.