Adult cardiomyocytes are irreversibly postmitotic but respond to a variety of stimuli by hypertrophic growth, which is associated with an increase in cell size and protein content, organization of sarcomeres, and activation of a fetal gene program. Recently, we described a novel cardiac helicase activated by MEF2 protein (CHAMP), which is expressed specifically in the heart throughout prenatal and postnatal development. Here we show that CHAMP acts as an inhibitor of cell proliferation and cardiomyocyte hypertrophy. Ectopic expression of CHAMP inhibits proliferation of HeLa cells and blocks cell cycle entry of serum-stimulated NIH 3T3 cells. Overexpression of CHAMP in primary neonatal cardiomyocytes blocks hypertrophic growth and the induction of fetal genes in response to stimulation by serum and phenylephrine but does not prevent sarcomere organization or early mitogenic signaling events including activation of extracellular signal-regulated kinases or up-regulation of c-fos. Inhibition of cardiomyocyte hypertrophy by CHAMP requires the conserved ATPase domain and is accompanied by up-regulation of the cyclin-dependent protein kinase inhibitor p21(CIP1). These findings identify CHAMP as a cardiac-specific suppressor of cardiomyocyte hypertrophy and cell cycle progression and suggest that CHAMP may suppress these processes through the regulation of p21(CIP1).