The induction and activation of autologous cytotoxic cells (lym-phokine activated killer cells = LAK) by interleukin-2 (Il –2) is an interesting new approach in cancer treatment. So far, Il –2 alone or in combination with the transfer of in vitro activated LAK (adoptive immunotherapy = AI) was shown to be effective predominantly in renal cell cancer and malignant melanoma with a response rate of 20–35%. The results in colorectal tumors are disappointing. Clinical experiences with Il –2 in other tumor entities are limited and/or mostly lack sufficient responses. To improve therapeutic results and to reduce the serious side effects, present trials focus on combinations of Il–2 with other cytokines, predominantly interferon-α (IFN-α), or chemotherapy. So far, the combination of Il –2 + IFN-α seems to be at least as effective as Il -2 +AI in renal cell cancer. Combinations of chemotherapy and Il –2, especially in gastrointestinal tumors, have not been shown to exceed the moderate results of chemotherapy alone so far. Trials with highly activated or specific cytotoxic cells as tumor-infiltrating lymphocytes (TILs) or adherent-LAK-cells are still more experimental. The value of IL-2 for elimination of minimal residual disease in acute leukemias after autologous bone marrow transplantation or as consolidation of complete response will have to be defined. The present paper reviews clinical studies with Il –2 in malignancies and its significance for ther