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Infarct size represents a powerful predictor of mortality and adverse events. The ability of 3D speckle-tracking to estimate infarct size in STEMI was not explored.Methods: 49 pts (age 60±15) with a first recent STEMI (8±3 days) successfully reperfused by primary PCI were studied. Peak global 2D longitudinal strain (LS) from 3 apical views (68±9 fps) and 3D LS, circumferential (CS), radial (RS) and area strain (AS) from 4-beat LV data sets (30±3 vps) were measured. Infarct size was estimated by peak cTnI levels in all pts. In 27 pts, delayed-enhancement MRI (DE-MRI) was performed within 24h from echo study. Transmural extent of necrosis (% DE) was measured and infarct size index (ISI, %) was calculated as the sum of %DE for all LV segments divided by 17.Results: Peak cTnI (mean 162 μg/L, range 2.0-916) was correlated with 2D LS and all 3D strains, as well as with 3D LV volumes and ejection fraction (Table). At multivariable analysis, only CS emerged as significant independent predictor of infarct size (β 0.584, p<0.001). In MRI pts, CS showed the closest correlation with ISI among all strains (r=0.716, p<0.001) and the best discriminative power to predict the extent of necrosis transmurality at DE-MRI (F ANOVA 82.1, p<0.0001). In 20 pts, intraclass correlation coefficient for 3D CS was 0.98 for intra- and 0.95 for interobserver reproducibility.Conclusion. In STEMI pts, 3D CS showed good reproducibility and accuracy for infarct size estimation in comparison with DE-MRI and cTnI, being superior than 2D and 3D LS. This is the first study demonstrating that 3D CS may be used as an objective marker of infarct size and necrosis transmurality at bedside.