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Plaque ruptures and attenuated plaques are considered to be unstable and have been identified in both culprit and nonculprit lesions of patients with ST-segment elevation myocardial infarction (STEMI). However, there are limited data available on the natural evolution of these plaques and their long-term clinical outcome. We investigated the natural evolution and long-term impact of plaque ruptures and attenuated plaques in untreated segments of infarct-related arteries in patients with STEMI.In the Harmonizing Outcomes with Revascularization and Stents in Acute Myocardial Infarction trial, 389 patients with 429 lesions underwent intravascular ultrasound (IVUS) at baseline. Follow-up IVUS at 13 months was conducted in 245 patients. Three-year follow-up data were available for all patients.Segments not treated between baseline and follow-up were compared. Baseline IVUS identified 29 plaque ruptures in 27 patients (7%). Of 11 plaque ruptures with follow-up IVUS, four healed and seven persisted. Conversely, through follow-up IVUS, nine new plaque ruptures in nine patients (4%) were identified. Attenuated plaques were identified in 31 of 38 plaque ruptures (81.5%), of which 24 were in the same circumferential segment as the ruptured cavity and seven were within 5 mm proximal or distal to the plaque rupture. Morphologic changes during follow-up, including new plaque ruptures and changes in the attenuated plaque frequency and distribution, were not accompanied by either serious lumen compromise or clinical events.Serial IVUS analysis demonstrated that the morphology of unstable plaques within untreated segments in STEMI patients treated with optimal systemic therapies markedly changed during the 13-month follow-up period, without lumen compromise or clinical events at the 3-year follow-up.