Myocardial infarction is commonly complicated by left ventricular remodeling, a process that leads to cardiac dilatation, congestive heart failure and death. The innate immune system plays a pivotal role in the remodeling process via nuclear factor (NF)-B activation. The NF-B transcription factor family includes several subunits (p50, p52, p65, c-Rel, and Rel B) that respond to myocardial ischemia. The function of NF-B p50, however, is controversial in this process. To clarify the role of NF-B p50 in postinfarct left ventricular remodeling, myocardial infarction was induced in wild-type 129Bl6 mice and NF-B p50–deficient mice. Without affecting infarct size, deletion of NF-B p50 markedly increased the extent of expansive remodeling (end-diastolic volume: 176±13 µL versus 107±11 µL; P=0.003) and aggravated systolic dysfunction (left ventricular ejection fraction: 16.1±1.5% versus 24.7±3.7%; P=0.029) in a 28-day time period. Interstitial fibrosis and hypertrophy in the noninfarcted myocardium was increased in NF-B p50 knockout mice. In the infarct area, a lower collagen density was observed, which was accompanied by an increased number of macrophages, higher gelatinase activity and increased inflammatory cytokine expression. In conclusion, targeted deletion of NF-B p50 results in enhanced cardiac remodeling and functional deterioration following myocardial infarction by increasing matrix remodeling and inflammation.