Background Early systolic lengthening (ESL) may occur in ischemic myocardial segments with reduced contractile force. We sought to evaluate the prognostic potential of ESL in patients with ST-segment-elevation myocardial infarction treated with primary percutaneous coronary intervention. Methods and Results We prospectively enrolled 373 patients with ST-segment-elevation myocardial infarction treated with primary percutaneous coronary intervention. All patients underwent a speckle tracking echocardiographic examination a median of 2 days (interquartile range, 1-3 days) after the percutaneous coronary intervention. We assessed a novel viability index, the ESL index, defined as follows: [-100×(peak positive systolic strain/peak negative strain in cardiac cycle)]. We also calculated ESL duration, defined as time from onset of QRS complex on the ECG to time of peak positive systolic strain. Both parameters were averaged from 18 myocardial segments. During a median follow-up of 5.3 years (interquartile range, 2.5-6.0 years), 145 (39%) experienced major adverse cardiovascular events, a composite of incident heart failure, new myocardial infarction, and all-cause mortality. The ESL index and ESL duration were significantly increased in culprit lesion areas (6.7±6.2% versus 5.0±4.1% and 43±33 ms versus 33±24 ms, respectively; P<0.001 for both). In Cox proportional hazard models, the ESL index (hazard ratio, 1.27 per 1% increase; 95% CI, 1.13-1.43; P<0.001) and ESL duration (hazard ratio, 1.49 per 1-ms increase; 95% CI, 1.15-1.92; P=0.002) yielded prognostic information on major adverse cardiovascular events. Both associations remained significant after adjusting for clinical, echocardiographic, and invasive confounders. Conclusions Assessment of ESL after primary percutaneous coronary intervention in patients with ST-segment-elevation myocardial infarction yields independent and significant prognostic information on the future risk of cardiovascular events.