miRNAs are non-coding, single-stranded RNAs that regulate target gene expression by repressing translation or promoting RNA cleavage. Dicer is an essential component of the miRNA processing machinery. To identify a role for miRNAs in tumorigenesis, we designed an adenovirus expressing small hairpin RNA (shRNA) to silence Dicer and globally suppress the maturation of miRNAs. We identified that the impairment of miRNA processing conferred an enhanced proliferative activity and invasive ability on each of three tumor cell lines in vitro. Inhibition of Dicer was associated with activation of p-Akt and enhanced expression of the cell cycle associating molecules, cyclin A and PCNA, as well as MMP-2 and MMP-9, proteins involved in tumor cell invasion. Adenoviral gene silencing of Dicer in subcutaneous MCF-7 xenografts significantly increased tumor growth in vivo compared to tumors infected with non-loading adenovirus. Increased tumor growth was associated with p-Akt activation and upregulation of cyclin A, PCNA MMP-2 and MMP-9. These findings demonstrate that global reduction of miRNA processing by silencing Dicer enhances tumor proliferation and invasion, and the p-Akt pathway may contribute to this phenotype via the downstream molecules, cyclin A, PCNA, MMP-2 and MMP-9.