Because systemic spread occurs early in the growth of many malignancies, control of occult micrometastases must be an integral part of cancer treatment. For this reason, surgery and radiation therapy alone may fail to achieve a cure despite eradication of the primary tumor. Chemotherapy is potent and systemic in its effects but kills tumor cells by first-order kinetics so the last cancer cell may not be eliminated. An agent is needed that can selectively attack and destroy small numbers of tumor cells on a systemic basis without a significant increase in toxicity. Experimental observations indicate that immunotherapy could fill this role. Immunotherapy has been tested as an adjuvant to surgery, radiation therapy, and chemotherapy, and is clearly beneficial for selected cancer patients. There are many unresolved questions regarding the underlying mechanisms as well as the practical application of adjuvant immunotherapy, but the initial investigations indicate that it could play a vital part in the treatment of cancer. There is evidence that stimulation of host resistance can result in control of systemic micrometastases.