NF kappa B (NF‐κB) is a ubiquitous transcription factor activated by various stimuli implicated in heart failure progression including reactive oxygen species (ROS), hypoxia, and inflammatory cytokines. Although NF‐κB is involved in ischemic preconditioning, unstable angina pectoris, and atherogenesis, its role in heart failure has not been determined. Therefore, we investigated left ventricular remodeling in mice with a targeted deletion of the NF‐κB subunit p50/NF‐κB1 after myocardial infarction. p50 knockout (KO) and wild‐type (WT) animals underwent coronary artery ligation. Transthoracic echocardiography was performed at days 0, 21, and 56 at midpapillary levels. Early mortality was significantly lower in KO than in WT animals. Moreover, p50 KOs exhibited significantly reduced ventricular dilatation over 8 wk compared to WT controls (end‐systolic diameters by transthoracic echocardiography, WT vs. KO, 0.55±0.04 vs. 0.34±0.03 cm) and preserved left ventricular contractility. Collagen content and matrixmetalloproteinase (MMP) ‐9 expression were significantly lower in KO mice after myocardial infarction and may account for improved left ventricular remodeling. Absence of the NF‐κB subunit p50 improves early survival and reduces left ventricular dilatation after myocardial infarction. NF‐κB might therefore be an attractive target to treat heart failure.—Frantz, S., Hu, K., Bayer, B., Gerondakis, S., Strotmann, J., Adamek, A., Ertl, G., Bauersachs, J. Absence of NF‐κB subunit p50 improves heart failure after myocardial infarction. FASEB J. 20, E1309–E1314 (2006)