OBJECTIVES: This study assessed changes in optical coherence tomography (OCT)-defined plaque composition in patients with ST-elevation myocardial infarction (STEMI) receiving high-intensity statin treatment. BACKGROUND: OCT is a high-resolution modality capable of measuring plaque characteristics including fibrous cap thickness (FCT) and macrophage infiltration. There is limited in vivo evidence regarding the effects of statins on OCT-defined coronary atheroma composition and no evidence in the context of STEMI. METHODS: In the IBIS-4 (Integrated Biomarker Imaging Study-4), 103 patients underwent intravascular ultrasonography and OCT of 2 noninfarct-related coronary arteries in the acute phase of STEMI. Patients were treated with high-dose rosuvastatin for 13 months. Serial OCT imaging was available in 153 arteries from 83 patients. We measured FCT by using a semi-automated method. Co-primary endpoints consisted of the change in minimum FCT (measured in fibroatheromas) and change in macrophage line arc. RESULTS: At 13 months, median low-density lipoprotein cholesterol had decreased from 128 mg/dl to 73.6 mg/dl. Minimum FCT, measured in 31 lesions from 27 patients, increased from 64.9 ± 19.9 μm to 87.9 ± 38.1 μm (p = 0.008). Macrophage line arc decreased from 9.6° ± 12.8° to 6.4° ± 9.6° (p < 0.0001). The secondary endpoint, mean lipid arc, decreased from 55.9° ± 37° to 43.5° ± 33.5°. In lesion-level analyses (n = 191), 9 of 13 thin-cap fibroatheromata (TCFAs) at baseline (69.2%) regressed to non-TCFA morphology, whereas 2 of 178 non-TCFA lesions (1.1%) progressed to TCFAs. CONCLUSIONS: In this observational study, we found significant increase in minimum FCT, reduction in macrophage accumulation, and frequent regression of TCFAs to other plaque phenotypes in nonculprit lesions of patients with STEMI treated with high-intensity statin therapy.