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BACKGROUND Cytokine-induced killer (CIK) cells are a heterogeneous group of immune cells that exert potent MHC-unrestricted cytotoxicity toward various cancer cells in both solid and hematological malignancies. OBJECTIVE The purposes of this study were to compare the expansion and characteristics of cytokine-induced killer cells between a standard culture method and a gas-permeable culture method and to develop a clinical-scale expansion protocol for cytokine-induced killer cells using a gas-permeable culture method. METHODS We compared the absolute cell number, fold change, cell subsets, activation markers, cytokine concentrations, and cytotoxicity toward myeloid leukemia cell lines between cytokine-induced killer cells expanded using two different culture methods. Then, we determined the ability to achieve clinical-scale expansion of cytokine-induced killer cells using the gas-permeable culture method. RESULTS Cytokine-induced killer cells in the gas-permeable culture method group exhibited significantly better expansion but maintained similar cell subsets, activation markers, and cytotoxicity to those in the standard culture method group. In addition, we successfully manufactured cytokine-induced killer cells for clinical use using the gas-permeable culture method. We also showed the clinical efficacy of allogeneic cytokine-induced killer cells produced by the gas-permeable culture method in a patient with acute myeloid leukemia that relapsed after allogeneic hematopoietic stem cell transplantation. This patient maintained ongoing disease remission for 2 years with minimal side effects after cytokine-induced killer cell infusion. CONCLUSIONS We successfully developed a simple and effective protocol for the ex vivo expansion of cytokine-induced killer cells using the gas-permeable culture method for clinical application.