Transforming growth factor-βs (TGF-βs) are pleiotropic cytokines involved in many physiological and pathological processes, including heart development and heart disease. Smad4 is the central intracellular mediator of TGF-β signaling. To investigate the function of Smad4 in heart development further, we generated a strain of cardiomyocyte-specific Smad4 knockout mice using the Cre–loxP system. Unexpectedly, the deletion of Smad4 in cardiomyocytes resulted in cardiac hypertrophy characterized by an increase in the size of cardiac myocytes, age-associated fibrosis, and reexpression of certain fetal genes. Approximately 70% of the Smad4 mutant mice died spontaneously between 5 and 12 months of age. Echocardiography and an invasive hemodynamic study of the left ventricle revealed markedly decreased cardiac contractility in Smad4 mutant mice compared with littermate controls. Moreover, phosphorylated extracellular signal–regulated kinase (ERK) 1/2 and mitogen-activated protein kinase–ERK (MEK) 1 were increased in the Smad4 mutants, suggesting that an upregulation of MEK1–ERK1/2 signaling as a consequence of deletion of Smad4 underlies the impaired cardiac function. These results reveal an important function of Smad4 in cardiac remodeling and suggest that an altered cellular response to TGF-β could be a mechanism by which cardiac myocytes undergo hypertrophy.