Objective— Although IL-18 has been implicated in atherosclerotic lesion development, little is known about its role in advanced atherosclerotic plaques. This study aims to assess the effect of IL-18 overexpression on the stability of preexisting plaques. Methods and Results— Atherosclerotic lesions were elicited in carotid arteries of apolipoprotein E (apoE)-deficient mice (n=32) by placement of a perivascular collar. Overexpression of IL-18 was effected by intravenous injection of an adenoviral vector 5 weeks after surgery. Two weeks after transduction, lesions were analyzed histologically with regard to plaque morphology and composition or by real-time polymerase chain reaction. No difference in plaque size was detected between groups. In the Ad.IL-18–treated group, 62% of lesions displayed a vulnerable morphology or even intraplaque hemorrhage as compared with only 24% in the controls ( P =0.037). In agreement, IL-18 overexpression reduced intimal collagen by 44% ( P <0.003) and cap-to-core ratio by 41% ( P <0.002). Although IL-18 did not affect the expression of collagen synthesis-related genes, it was found to enhance the collagenolytic activity of vascular smooth muscle cells in vitro, suggesting that the low collagen content is attributable to matrix degradation rather than to decreased synthesis. Conclusion— Systemic IL-18 overexpression markedly decreases intimal collagen content and cap thickness, leading to a vulnerable plaque morphology.