Natural killer (NK) cells are gaining growing attention due to their promise for immunotherapy. A fast and accurate system is needed to test NK cell biology and their therapeutic application. Here, we report a lung cancer organoid-based system to evaluate NK cells' cytotoxicity. We first established the lung cancer organoids on top of Matrigel, which allows the co-culture with NK cells. When co-cultured, NK cells moved close to and inside the lung cancer organoids. When we analyzed by flow cytometry, co-culture of NK cells induced a significantly higher ratio of cell death of lung cancer organoids, suggesting that lung cancer organoids can be employed to test the cytotoxicity of NK cells. Finally, the pre-treatment of NK cells with A83-01, a TGF