Background: Interleukin (IL)-33 is a member of interleukin (IL)-1 family of cytokines which has been linked to the development of inflammatory conditions and cancer in the gastrointestinal tract. This study is designed to investigate whether IL-33 has direct effect on human gastric epithelial cells (GES-1) and on human gastric adenocarcinoma cell line (AGS) and gastric carcinoma cell line (NCI-N87), assessing its role in regulation of cell proliferation and cell cycle, apoptosis and necrosis. Cell cycle regulation was also determined in ex vivo gastric cancer samples obtained during endoscopy and surgical procedures. Methods: cell lines and tissue samples underwent stimulation with rhIL-33. Proliferation was assessed by XTT and CFSE assay, we also evaluated apoptosis by Caspase 3/8 Activity assay and Annexin V assays. Cell cycle were analyzed by means of Propidium Iodine assay and gene expression regulation was assessed by RT-PCR Profiling. Results: we found that IL-33 has an antiproliferative and proapoptotic effect on cancer cell line, while it can stimulate proliferation and reduce apoptosis in normal epithelial cell line. These effects are also confirmed by the analysis of cell cycle gene expression which showed a reduced expression of proproliferative genes in cancer cells, in particular genes involved in G0/G1 and G2/M checkpoint. These results are confirmed by the gene expression analysis on surgical and bioptic specimens. Conclusions: the aforementioned results indicate that IL-33 may be involved in cell proliferation in an environment- and cell type-dependent fashion.