There are substantial evidences indiacting that anesthesia and surgery produce significant suppression of immune function. It is generally recognized that immunosuppressive manifestations such as lymphopenia, granulocytosis, decreased transformation to various antigens and impaired neutrophil chemotaxis could be followed by surgical operation. anesthetic agents may also have both direct and hormone-mediated indirect effects on immune systems. Recent evidences indicate that immunosurveillance mechanisms are not only important in the management of emerging infection and malignancy, but also critical to successful elimination of microscopic residual tumor after operation. This study was done to investigate the effeets of halothane anesthesia on T-lymphocyte subpopulations in patients undergoing hysterectomy. Lymphocyte, granulocytes and T-lymphoeyte subpopulations were counted before, during and after anestesia. The results showed that the decrease in the CD4+ cells (helper/inducer T-cells) was bigger than the decrease in the CD8+ cells (suppressor/ cytotoxic T-cells). CD3+ cells (total T-cells) and CD45R+ cells were also decreased. However, these decrease turned to normal values in 5 days. The overall result of this study suggest that anesthesia with halothane may bring temporal suppressive effect on the immunologic responses.