Lung cancer is the leading cause of cancer deaths worldwide, and current therapies fail to treat this disease in the vast majority of cases. The RAS and p53 pathways are two of the most frequently altered pathways in lung cancers, with such alterations resulting in loss of responsiveness to current therapies and decreased patient survival. The microRNA-34 (mir-34) gene family members are downstream transcriptional targets of p53, and miR-34 expression is reduced in p53 mutant tumors; thus, we hypothesized that treating mutant Kras;p53 tumors with miR-34 would represent a powerful new therapeutic to suppress lung tumorigenesis. To this end we examined the therapeutically resistant Kras^LSL-G12D/+;Trp53^LSL-R172H/+ mouse lung cancer model. We characterized tumor progression in these mice following lung-specific transgene activation and found tumors as early as 10 weeks postactivation, and severe lung inflammation by 22 weeks. Tumors harvested from these lungs have elevated levels of oncogenic miRNAs, miR-21 and miR-155; are deficient for p53-regulated miRNAs; and have heightened expression of miR-34 target genes, such as Met and Bcl-2. In the presence of exogenous miR-34, epithelial cells derived from these tumors show reduced proliferation and invasion. In vivo treatment with miR-34a prevented tumor formation and progression in Kras^LSL-G12D/+;Trp53^LSL-R172H/+ mice. Animals infected with mir-34a-expressing lentivirus at the same time as transgene activation had little to no evidence of tumorigenesis, and lentivirus-induced miR-34a also prevented further progression of preformed tumors. These data support the use of miR-34 as a lung tumor-preventative and tumor-static agent. Cancer Res; 72(21); 5576–87. ©2012 AACR.