AIM: The study of immune response of open versus laparoscopical total mesorectal excision with anal sphincter preservation in patients with rectal cancer has not been reported yet. The dissected retroperitoneal area that contacts directly with carbon dioxide is extensive in laparoscopic total mesorectal excision with anal sphincter preservation surgery. It is important to clarify whether the immune response of laparoscopic total mesorectal excision with anal sphincter preservation (LTME with ASP) in patients with rectal cancer is suppressed more severely than that of open surgery (OTME with ASP). This study was designed to compare the immune functions after laparoscopic and open total mesorectal excision with anal sphincter preservation for rectal cancer. METHODS: This study involved 45 patients undergoing laparoscopic (n=20) and open (n=25) total mesorectal excisions with anal sphincter preservation for rectal cancer. Serum interleukin-2 (IL-2), interleukin-6 (IL-6), tumor necrosis factor a (TNFa) were assayed preoperatively and on days 1 and 5 postoperatively. CD3+ and CD56+ T lymphocyte count, CD3-and CD56+ natural killer cell (NK) count and immunoglobulin (IgG/IgM/IgA) were assayed preoperatively and on day 5 postoperatively. The numbers of CD3+ and CD56+ T lymphocytes and CD3- and CD56+ NK cells were counted using flow cytometry. An enzyme-linked immunosorbent assay (ELISA) was used for IL-2, IL-6 and TNFa determination. And IgG, IgM, and IgA were assayed using immunonephelometry. RESULTS: The demographic data of the two groups had no difference. The preoperative levels of CD3+ and CD56+ T lymphocyte count, CD3- and CD56+ NK count, serum IgG, IgM, IgA, IL-2, IL-6 and TNFa also had no significant difference in the two groups (P>0.05). The CD3+ and CD56+ T lymphocyte counts had no obvious changes after surgery in laparoscopic (d=-0.79