In cells capable of entering the cell cycle, including cancer cells, beta-catenin has been termed a master switch, driving proliferation over differentiation. However, its role as a transcriptional activator in terminally differentiated cells is relatively unknown. Herein we utilize conditional, cardiac-specific deletion of the beta-catenin gene and cardiac-specific expression of a dominant inhibitory mutant of Lef-1 (Lef-1Delta20), one of the members of the T-cell factor/lymphocyte enhancer factor (Tcf/Lef) family of transcription factors that functions as a coactivator with beta-catenin, to demonstrate that beta-catenin/Tcf/Lef-dependent gene expression regulates both physiologic and pathological growth (hypertrophy) of the heart. Indeed, the profound nature of the growth impairment of the heart in the Lef-1Delta20 mouse, which leads to very early development of heart failure and premature death, suggests beta-catenin/Tcf/Lef targets are dominant regulators of cardiomyocyte growth. Thus, our studies, employing complementary models in vivo, implicate beta-catenin/Tcf/Lef signaling as an essential growth-regulatory pathway in terminally differentiated cells.