Objectives: This study sought to assess the effect of short-term apolipoprotein (apo) A-IMilano administration on plaque size and on suspected markers of plaque vulnerability. Background: Long-term lipid-lowering interventions can regress and stabilize atherosclerotic plaques. However, the majority of recurrent events occur early after the first episode. Interventions able to acutely induce plaque regression and stabilization are lacking. Regression of human coronary lesions after 5 weeks of treatment with apoA-IMilano administration has been shown. However, there are no data regarding its effect on plaque vulnerability. Methods: Advanced aortic lesions were induced in New Zealand White rabbits (n = 40). Plaque size was assessed by magnetic resonance imaging (MRI) at the end of atherosclerosis induction. Animals were randomized to placebo or apoA-IMilano phospholipids (ETC-216), 2 infusions 4 days apart. After the last dose, another MRI study was performed and aortas were processed for cellular composition and gene protein expression of markers associated with plaque instability. Results: Pre-treatment MRI showed similar plaque size in both groups, whereas post-treatment MRI showed 6% smaller plaques in apoA-IMilano-treated animals compared with placebo (p = 0.026). The apoA-IMilano treatment induced a 5% plaque regression (p = 0.003 vs. pre-treatment), whereas the placebo showed no significant effect. Plaque regression by apoA-IMilano was associated with a reduction in plaque macrophage density and a significant down-regulation in gene and protein expression of tissue factor, monocyte chemoattractant protein-1, and cyclooxygenase-2, as well as marked decrease in gelatinolytic activity. Conversely, cyclooxygenase-1 was significantly up-regulated. Conclusions: Acute plaque regression observed after short-term apoA-IMilano administration was associated with a significant reduction in suspected makers of plaque vulnerability in an experimental model of atherosclerosis.