In many types of cancer, presence of eosinophils in tumors correlate with an improved disease outcome. In line with this, activated eosinophils have been shown to reduce tumor growth in colorectal cancer (CRC). Interleukin (IL)-33 has recently emerged as a cytokine that is able to inhibit the development of tumors through eosinophils and other cells of the tumor microenvironment thereby positively influencing disease progress. Here, we asked whether eosinophils are involved in the effects of IL-33 on tumor growth in CRC.In models of CT26 cell engraftment and colitis-associated CRC, tumor growth was reduced after IL-33 treatment. The growth reduction was absent in eosinophil-deficient ΔdblGATA-1 mice but was restored by adoptive transfer of ex vivo-activated eosinophils indicating that the antitumor effect of IL-33 depends on the presence of eosinophils. In vitro, IL-33 increased the expression of markers of activation and homing in eosinophils, such as CD11b and Siglec-F, and the degranulation markers CD63 and CD107a. Increased expression of Siglec-F, CD11b and CD107a was also seen in vivo in eosinophils after IL-33 treatment. Viability and cytotoxic potential of eosinophils and their migration properties toward CCL24 were enhanced indicating direct effects of IL-33 on eosinophils. IL-33 treatment led to increased levels of IL-5 and CCL24 in tumors.Our data show that the presence of eosinophils is mandatory for IL-33-induced tumor reduction in models of CRC and that the mechanisms include eosinophil recruitment, activation and degranulation. Our findings also emphasize the potential use of IL-33 as an adjuvants in CRC immunotherapy. Abbreviations: AOM: azoxymethane; bmRPMI: bone marrow RPMI; CRC: colorectal cancer; CFSE: carboxyfluorescein succinimidyl ester; DSS: dextran sulfate sodium; EPX: eosinophil peroxidase; INF-γ: interferon gamma; ILC: innate lymphoid cell; IL-33: interleukin-33; IL-5: interleukin-5; MDSC: myeloid derived suppressor cells; NK cells: natural killer cells; P/S: penicillin/streptomycin; rm: recombinant mouse; T regs: regulatory T cells; TATE: tumor associated tissue eosinophilia; TNF-α: tumor necrosis factor alpha.Keywords: CT26; Eosinophils; adoptive transfer; colorectal cancer; interleukin-33.