MicroRNAs (miRNAs) are a recently discovered class of ∼22-nucleotide regulatory RNAs that post-transcriptionally regulate gene expression. We have recently demonstrated that muscle-specific miRNAs miR-1 and miR-133 play an important role in modulating muscle proliferation and differentiation. Here, we investigate the involvement of miRNAs in cardiac hypertrophy. We analyzed the global expression of miRNAs in agonist-induced hypertrophic cardiomyocytes as well as in pressure overload-induced hypertrophic hearts and found the miRNA expression profile altered in those hypertrophic conditions. We further show that inhibition of endogenous miR-21 or miR-18b augments hypertrophic growth. Conversely, introduction of functional miR-21 or miR-18b into cardiomyocytes represses myocyte hypertrophy. Together, our studies point to miRNAs as critical regulators of cardiac hypertrophy.