Background The association of plaques with outward arterial remodeling and acute coronary syndromes (ACS) has been mostly investigated by cross-sectional intravascular ultrasound studies. Magnetic resonance imaging (MRI) has made it possible to noninvasively assess the coronary vessels of patients with chronic coronary artery disease, but no study has been done in patients with ACS. We sought to serially investigate changes in coronary vessel walls of patients with ACS using noninvasive serial MRI. Methods A total of 42 segments of coronary arteries from 22 patients presenting with non-ST-segment elevation ACS were studied at baseline in the acute phase and at 6 months after stabilization and optimization of medical therapy. Patients received routine medical treatment during this period with control of risk factors. Vessel wall area, maximum wall thickness, mean wall thickness, and lumen area were analyzed longitudinally using MRI. Results Vessel wall area (38.8 +/- 20.0 vs 27.7 +/- 10.4 mm(2); P = .001), maximum wall thickness (2.9 +/- 0.7 vs 2.5 +/- 0.6 mm; P < .001), and mean wall thickness (2.0 +/- 0.7 vs 1.6 +/- 0.5 mm; P < .001) were significantly reduced at 6 months compared with baseline, whereas lumen area did not show significant changes (11.5 +/- 4.8 vs 10.9 +/- 5.0 mm(2); p = .52). The wall/lumen ratio was significantly reduced from 3.7 +/- 1.7 to 2.9 +/- 1.3 (P = .01), suggesting a regression of outward remodeling. Conclusion Patients with ACS have increased coronary vessel wall thickness and area that can regress with stabilization and medical therapy over the period of 6 months. Magnetic resonance imaging can detect and serially follow these changes, monitoring coronary vascular remodeling from the acute to the chronic phase of the disease.152611231132