The observed phenomenon that multiple distant metastases may appear and grow rapidly after operation on the primary cancer is very distressing. Many experimental results suggest that surgical procedures may precipitate dissemination and growth of tumor in some instances, but the overwhelming evidences document that surgical reduction of tumor bulk can achieve cure for the host and restore the immunity lost in the face of growing tumors. Various anesthetics were shown to interfere with many phases of the immune response. But recent studies suggest that the inhibitory effect of anesthesia alone is minimal. Depression of lymphocyte transformation, detectable as early as 2 hours after induction, was related primarily to the extent of tissue trauma, the amount of blood loss, duration of operation, and whether thoracic or abdominal cavity was entered. Posoperative changes of lymphocyte counts and transformation responses usually returned to normal values within a week, whereas depression of specific cellular immunity to tumor-associated antigen in vitro, and delayed cutaneous hypersensitivity reactions in vivo, persisted for about a week and gradually returned to normal by 3 weeks. Presently the clinical significance of such transitory depression of host immunity is not known. It is hoped that this review may stimulate interest in further experimental and clinical research.