Background: Double negative T cell (DNT) is defined by the expression of CD3 (or TCRαβ) with the absence of CD4, CD8 and iNKT cell markers. DNTs account for 1-3% of the total peripheral lymphocytes. Although healthy donor cells derived allogeneic DNTs were reported to have anti-tumor effects in several cancer models, the function of DNTs in tumor microenvironment is not well understood. In this study, we aim to characterize DNTs in cancer tissues and lymph nodes of colorectal cancer patients including their immunological functions. Methods: A total of 14 colorectal cancer tissues and regional lymph nodes were collected and examined for the proportion and the immunological functions of DNTs by FACS analysis. Results: DNTs, which were defined in this study as TCRαβ positive with CD4-, CD8- and CD56- T cells, were detected in colorectal cancer tissues. DNT cells account for more than 5% of TCRαβ-positive populations in 12 cases and out of which 7 cases were more than 20% observed in this study. In all of these 7 cases, DNT cells were also found to be higher in the tumor tissues than in non-metastatic lymph nodes. In addition to colorectal cancer, DNTs were found to be more than 5% of TCRαβ-positive populations in cancer tissues in 3 out of the 6 cases of ovarian and uterine cancer cases.To further evaluate the potential immunological functions of DNTs, expression of cell membrane protein markers were evaluated. CD3ε expression was down regulated in almost all of the DNTs. The CD107a expression of DNTs were higher than that of CD8⁺ T cells in tumor tissues. When co-culturing DNTs with tumor cells, CD107a expression of DNTs was significantly upregulated. Notably, the intracellular production of perforin and granzyme B were lower than CD8⁺ T cells. Fas-L expression were found to be higher on DNT cells compared to other T cell populations, suggesting DNT might possess cytotoxic activity in the tumor microenvironment. It is known that upregulation of Fas expression is caused by T cell activation and subsequently induces T cell apoptosis. The upregulation of Fas expression was also observed in DNT cells from non-metastatic lymph nodes. In contrast, Fas expression of DNT cells in tumor tissues was observed to be lower than CD4⁺ or CD8⁺ T cells, suggesting that DNT could escape from apoptosis. This downregulation of Fas expression was speculated to be one of the reasons for the accumulation of DNTs in tumor tissues. Conclusions: Although the function of DNTs was still not well elucidated, the accumulation of DNT cells in tumor microenvironment suggests DNTs could be a potential target for cancer immunotherapy.