The failing human heart expresses tumor necrosis factor-α (TNF-α). However, its pathophysiological significance is not clear. We previously reported that robust overexpression of TNF-α in the murine heart causes lethal myocarditis. In this study, we modified the transgene to reduce the production of TNF-α by preserving the destabilizing sequence in TNF-α cDNA. Expression was driven by the murine α-myosin heavy chain promoter. Use of this modified construct allowed us to establish a murine transgenic line (TG). TG offspring were examined at 6, 12, and 24 weeks. All showed a significantly higher heart weight–to–body weight ratio. Northern blot analysis confirmed the expression of transgene in the heart, and enzyme-linked immunosorbent assay demonstrated the presence of TNF-α protein. The TG heart demonstrated a mild, diffuse, lymphohistiocytic interstitial inflammatory infiltrate. Cardiomyocyte necrosis and apoptosis were present but not abundant. Magnetic resonance imaging showed that the TG heart was significantly dilated with reduced ejection fraction. Although the left ventricular dP/dt_max was not different at baseline, its responsiveness to isoproterenol was significantly blunted in TG. Atrial natriuretic factor was expressed in the TG ventricle. A group of TG died spontaneously, and subsequent autopsies revealed exceptional dilatation of the heart, increased lung weight, and pleural effusion, suggesting that they died of congestive heart failure. The cumulative mortality rate at 6 months was 23%. In conclusion, the mouse overexpressing TNF-α recapitulated the phenotype of congestive heart failure. This provides a novel model to elucidate the role of this cytokine in the development of congestive heart failure.