The proinflammatory cytokine interleukin (IL)-1 signals exclusively through the type I IL-1 receptor (IL-1RI). IL-1 expression is markedly induced in the infarcted heart; however, its role in cardiac injury and repair remains controversial. We examined the effects of disrupted IL-1 signaling on infarct healing and cardiac remodeling using IL-1RI(-/-) mice. After reperfused infarction IL-1RI-null mice exhibited decreased infiltration of the infarcted myocardium with neutrophils and macrophages and reduced chemokine and cytokine expression. In the absence of IL-1 signaling, suppressed inflammation was followed by an attenuated fibrotic response. Infarcted IL-1RI(-/-) mice had decreased myofibroblast infiltration and reduced collagen deposition in the infarcted and remodeling myocardium. IL-1RI deficiency protected against the development of adverse remodeling; however, infarct size was comparable between groups suggesting that the beneficial effects of IL-1RI gene disruption were not attributable to decreased cardiomyocyte injury. Reduced chamber dilation in IL-1RI-null animals was associated with decreased collagen deposition and attenuated matrix metalloproteinase (MMP)-2 and MMP-3 expression in the peri-infarct area, suggesting decreased fibrotic remodeling of the noninfarcted heart. IL-1beta stimulated MMP mRNA synthesis in wild-type, but not in IL-1RI-null cardiac fibroblasts. In conclusion, IL-1 signaling is essential for activation of inflammatory and fibrogenic pathways in the healing infarct, playing an important role in the pathogenesis of remodeling after infarction. Thus, interventional therapeutics targeting the IL-1 system may have great benefits in myocardial infarction.