History and Objectives: Since breast cancer in the most prevalent type of spontaneous tumor in humans and defense against such tumors are usually mediated through natural killer (NK) cells and with their establishment and spread of the tumor especially breast cancer, the activity of natural killer cells are reduced and in order to determined the effect of mastectomy on the activity of natural killer cells, the present study is carried out on patients with breast cancer referred to the Imam Khomeini hospital in 1995.

Materials and Methods: A clinical trial was carried out to assess the effect of surgery on 18 patients. Cytotoxic effect of NK cells were assess before and after the surgery in mononuclear suspensions by flow cytometry. The activity of NK cell was determined among healthy individuals as control.

Results: Activity of NK cell increases from 24.4±12.6 percent to 37.7±22.3 before and after surgery respectively (P<0.05). Increasing the stage of the disease will reduce the activity of the NK cells (r=-0.89). However there were no significant differences before and after the surgery was observed in the same stage. There were relatively moderate correlation between age and NK cell activity (r=-0.53). Activity of NK cells was 62.5±3.5 percent in control group, which displayed significant difference (P<0.0001).

Conclusion: Mastectomy may lead to increased activity of NK cells among patient suffering from breast cancer and their increased activity may produce positive therapeutic effect.