ST2 (also known as IL1RL1) is the critical functional receptor for interleukin (IL)-33 in stimulating regulatory T cell (Treg) expansion and function in physiological and pathological conditions. We examined the correlation between ST2 cell expression and FoxP3 positive Tregs in both colorectal adenoma and cancer (CRC) microenvironment by real-time PCR, immunohistochemistry (IHC) and double immunofluorescences. The clinicopathological and prognostic significance of cellular ST2positive cells and FoxP3-positive Tregs in patients with adenoma and CRC were evaluated. Real-time PCR results revealed increased expression levels of ST2 and FoxP3 mRNAs in both adenoma and CRC tissues as compared with control tissues. IHC analysis confirmed increased densities of ST2-positive cells in both the adenoma/CRC epithelium and stroma, which show a close positive linear association with the densities of FoxP3-positive Tregs in respective compartments. Pathological feature analysis showed that densities of ST2-positive cells in the tumor stroma were notably associated with degree of dysplastic grading in patients with adenoma, and disease stages and lymph node metastasis in patients with CRC. Kaplan-Meier survival curves suggested that CRC patients with high densities of ST2-positive cells in the stroma tend to have a shorter overall survival. We therefore concluded that increased densities of ST2-postive cells relate to Treg accumulation within the adenoma/CRC microenvironment, suggesting the IL-33/ST2 pathway as a potential contributor for immunosuppressive milieu formation that impact disease stage and prognosis in patients with CRC. Colorectal cancer (CRC) is one of the most common malignant diseases worldwide. The progression of CRC is not only determined by CRC cells themselves but also by their immune microenvironment 1. Although high densities of immune cells have been observed in the CRC microenvironment, most CRCs still progress invasively with the development of metastasis. Most likely, these patients do not develop a satisfactory antitumor immune capacity and some CRC cells have escaped the immune surveillance control 2. Indeed, current scientific data strongly suggest that an immunosuppressive network is established in the CRC microenvironment wherein immune suppressive cells and cytokines play a critical role in inhibiting the host effective antitumor function, thereby promoting CRC progression and metastasis 3-5. Therefore, there is a great interest in studying the contributing factors of immunosuppressive microenvironment formation during CRC development 6,7. Interleukin (IL)-33, which is a member of the IL-1 cytokine family, regulates a Th2 response but also a Th1 response through natural killer cells, CD8 T cells and γδ T cells 8-11 and contributes to the generation of immunosuppressive cells 12-15. Current evidence regarding the role of IL-33 in the CRC development suggest that IL-33 is involved in the pathogenesis of CRC 1,16-18 , and IL-33 significantly promotes the establishment and progression of CRC in both...