Cytokines and inflammation have been implicated in the pathogenesis of heart failure. For example, IL-6 family cytokines and the gp130 receptor play important roles in cardiac myocyte survival and hypertrophy. Signal transducer and activator of transcription 3 (STAT3) is a major signaling protein that is activated through gp130. We have created mice with a cardiomyocyte-restricted deletion of STAT3. As measured by serial echocardiograms, mice with cardiac specific deletion of STAT3 are significantly more susceptible to cardiac injury after doxorubicin treatment than age-matched controls. Intriguingly, STAT3 appears to have a critical role in protection of inflammation-induced heart damage. STAT3-deficient mice treated with lipopolysaccharide demonstrated significantly more apoptosis than their WT counterparts. At the cellular level, cardiomyocytes with STAT3 deleted secrete significantly more tumor necrosis factor ␣ in response to lipopolysaccharide than those with WT STAT3. Furthermore, histologic examination of the cardiomyocyte-restricted STAT3-deficient mice reveals a dramatic increase in cardiac fibrosis in aged mice. Although no overt signs of heart failure are present in young STAT3-deficient mice, they spontaneously develop heart dysfunction with advancing age. These results indicate the crucial functions of STAT3 in cardiomyocyte resistance to inflammation and other acute injury and in pathogenesis of age-related heart failure.