Active immunotherapy of cancer is based on the hyperactivation of the immune system of tumor-bearing patients. Interleukin 2 (IL2) has proven to be the most potent immunostimulant used so far. Therapeutic activity of IL2 and IL2-activated lymphocytes has been reported in the treatment of certain tumors such as metastatic melanoma and renal carcinoma. Effectiveness is related to the dose of IL2 administered to the patients, as are important toxic effects. Other cytokines, such as interferon (IFN) and tumor necrosis factor (TNF), are used for their direct effect on malignant cells, particularly those of the lymphoid lineage. The development of these new immunotherapeutic approaches will depend on the quality of their scientific bases, the clinical follow-up of the toxic effects, and the social management of the costs.