BACKGROUND: In the acute phase of myocardial infarction (MI), infarct size and microvascular obstruction (MVO) are important prognostic factors for cardiovascular outcome. MI size is a major determinant of myocardial function, but the specific effect of MVO is less documented. The aim of this study was to evaluate the impact of MVO on longitudinal myocardial strain assessed by speckle-tracking echocardiography. METHOD: Speckle-tracking echocardiography and contrast-enhanced cardiac magnetic resonance studies were performed in 69 patients 72 hours after first acute MI. Segmental and global longitudinal systolic strain (epsilonL) was measured using speckle-tracking echocardiography. Transmural extent of MI, MI size, and the presence or absence of MVO were assessed using contrast-enhanced cardiac magnetic resonance. Left ventricular (LV) ejection fraction was assessed at 6 months using echocardiography. RESULTS: The mean infarct size was 23 +/- 13% of LV mass. MVO was present in 64% of patients. MVO was significantly associated with epsilonL impairment (-7.8 +/- 4.9% vs -16.3 +/- 6.4%, P \textless .001), and epsilonL remained significantly worse in MVO-positive segments after adjustment for transmural extent of MI. A epsilonL value \textgreater -12.5% predicted the presence of MVO with 83% sensitivity and 75% specificity. On multivariate analysis, global epsilonL and MI size, but not MVO, were identified as independent predictors of LV ejection fraction at follow-up (beta = -0.9, P = .023, and beta = -0.2, P = .034, respectively). CONCLUSION: In the acute phase of MI, segmental and global epsilonL is significantly altered by the presence of MVO, in addition to MI size. However, MI size but not MVO independently predicts LV ejection fraction at follow-up.