Abstract The delivery of microRNA (miRNA) via inhalation is a potential strategy for lung cancer prevention in high risk individuals such as current and former smokers. Previous studies have shown that intratracheal or intranasal exposure of genetically engineered mice to viral vectors expressing let-7 miRNA resulted in a reduced lung tumor burden. However, these studies were done in genetically engineered mouse models that developed highly aggressive tumors more relevant for a treatment regimen. In this study, we investigated the efficacy of let-7b in lung cancer prevention. An aerosolized let-7b miRNA formulation was tested in the benzo(a)pyrene (B[a]P) mouse model. The particle size of the let-7b miRNA aerosol has been systematically characterized as particles with sizes between 1.9 to 35 nm (average size of 29.0 nm). Following the aerosol formulation evaluation, the biodistribution of let-7b in the lung by tail vein injection and aerosolized delivery were compared. The distribution of the aerosolized let-7b produced higher levels of the miRNA in the lung compared with systematic delivery. The aerosol delivery of let-7b had no effect on animal body weight nor were there any other signs of toxicity. The development of an aerosolized let-7b formulation inhibited B[a]P - induced lung adenoma volume by 72% in A/J mice. Collectively, these findings show that aerosolized miRNA is a promising approach for lung cancer prevention. Citation Format: Qi Zhang, Jing Pan, Yian Wang, Mark S. Miller, Alberto Izzotti, Ming You. Chemoprevention of aerosolized Let-7b in mouse lung cancer model [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2540.