The　instantaneous　wave-free　ratio　(iFR)　closely　related　to　fractional　flow　reserve　(FFR)　is　a　adenosine-independent　physiologic　index　of　coronary　stenosis　severity.　We　sought　to　evaluate　whether　iFR　derived　from　coronary　computed　tomographic　angiography　(iFRCT)　can　be　used　as　a　novel　noninvasive　method　for　diagnosis　of　ischemia-causing　coronary　stenosis.　We　retrospectively　enrolled　33　patients　(47　lesions)　with　coronary　artery　disease　(CAD)　and　examined　with　coronary　computed　tomographic　angiography　(CTA),　invasive　coronary　angiography　(ICA),　and　FFR.　Patient-specific　anatomical　model　of　the　coronary　artery　was　built　by　original　resting　end-diastolic　CTA　images.　Based　on　the　model　and　computational　fluid　dynamics,　individual　boundary　conditions　were　set　to　calculate　iFRCT　as　the　mean　pressure　distal　to　the　stenosis　divided　by　the　mean　aortic　pressure　during　the　diastolic　wave-free　period　of　rest　state.　Ischemia　was　assessed　by　an　FFR　of　up　to　0.8,　while　anatomically　obstructive　CAD　was　defined　by　a　stenosis　of　at　least　50%　by　ICA.　The　correlation　between　iFR(CT)　and　FFR　was　evaluated.　The　receiver　operating　characteristic　(ROC)　curve　was　used　to　select　the　cut-off　value　of　iFRCT　for　diagnosis　of　ischemia-causing　stenosis.　The　diagnostic　performances　of　iFR(CT),　coronary　CTA,　and　iFR(CT)　plus　CTA　for　ischemia-causing　stenosis　were　compared　with　ROC　curve　and　Delong　method.　On　a　per-vessel　basis,　iFR(CT)　and　FFR　had　linear　correlation　(r=　0.75,　p＜　0.01).　ROC　analysis　identified　an　optimal　iFRCT　cut-off　value　of　0.82　for　categorization　based　on　an　FFR　cut-off　value　0.8,　and　the　diagnostic　accuracy,　sensitivity,　specificity,　positive　predictive　value　(PPV),　and　negative　predictive　value　(NPV)　of　iFR(CT)　were　78.72%,　70.59%,　83.33%,　70.59%,　and　83.33%,　respectively.　Compared　with　obstructive　CAD　diagnosed　by　coronary　CTA　(AUC=　0.60),　iFR(CT)　yielded　diagnostic　improvement　over　stenosis　assessment　with　AUC　increasing　from　0.6　by　CTA　to　0.87　(P＜　0.01)　and　0.90　(P＜　0.01)　when　iFRCT　plus　CTA.　In　conclusion,　iFR(CT)　is　a　promising　index　improving　diagnostic　performance　over　coronary　CTA　for　detection　of　ischemia-causing　coronary　stenosis.　Abbreviations:　AUC　=　area　under　the　receiver　operating　characteristic　curve,　BMI　=　body　mass　index,　CAD　=　coronary　artery　disease,　CFD　=　computational　fluid　dynamics,　CTA　=　computed　tomography　angiography,　FFR　=　fractional　flow　reserve,　ICA　=　invasive　coronary　angiography,　iFR　=　instantaneous　wave-free　ratio,　iFR(CT)　=　instantaneous　wave-free　ratio　derived　from　coronary　computed　tomography　angiography,　LAD　=　left　anterior　descending,　LCX　=　left　circumflex,　NPV　=　negative　predictive　value,　PPV　=　positive　predictive　value,　RCA　=　right　coronary　artery,　ROC　=　receiver　operating　characteristic　curve.