Postinfarction mechanical dispersion (MD), that is, the regional heterogeneity of myocardial contraction throughout the cardiac cycle, has detrimental effects on left ventricular (LV) function and is related to the occurrence of heart failure and ventricular arrhythmias. However, its prevalence, pathophysiological determinants, and clinical utility are still unknown. The aim of the present study is to clarify these issues. In total, 130 consecutive patients (mean age 60 ± 12 years, 75% male) with a first ST-segment elevation myocardial infarction (STEMI) were included. Cardiac magnetic resonance (CMR) with late gadolinium enhancement imaging was performed to assess LV function, infarct size, and microvascular obstruction. Feature-tracking analysis was applied to cine-CMR short-axis images to assess MD, defined as the SD of the time-to-peak circumferential strain of the LV segments expressed as percent cardiac cycle. For comparison purpose, 40 control subjects similar in age and gender to the STEMI group were also included. Patients were followed-up for a median of 95 months; the outcome event was defined as a composite of cardiovascular death, aborted sudden cardiac death, and hospitalization for heart failure. STEMI patients had significantly higher MD compared with controls (12.0 ± 5.35% vs 3.85 ± 0.99%, p <0.001). At multivariate analysis, heart rate (β = 0.20, p = 0.008), LV end-systolic volume index (β = 0.37, p <0.001), and infarct size (β = 0.23, p = 0.017) were significantly and independently related to MD. The outcome event occurred in 26 (20%) patients. At multivariate Cox proportional hazards analysis, MD was significantly and independently related to the outcome event (p <0.001). MD provided significant incremental value over the other clinical and CMR variables in predicting the outcome event (p <0.001 for the chi-square change). In conclusion, MD after STEMI is a marker of the extent of myocardial damage; its assessment by feature-tracking CMR provides significant, independent, and incremental long-term prognostic information.