Modern immunotherapy of human cancer has evolved as a rapidly expanding field of clinical and experimental research. Employing the systemic application of recombinant interleukin-2 (IL-2) in humans, Rosenberg and colleagues from the National Cancer Institute reported the regression of advanced metastatic tumors in approximately 10%-30% of patients treated. The additional adoptive transfer of autologous patient-derived activated lymphocytes was performed to enhance therapeutic efficacy. While the exact mechanisms of IL-2 based immunotherapy in cancer remain unclear, it has been hypothesized that both the IL-2 activated lymphocyte and its secretory products such as interferon-gamma or tumor-necrosis factor beta may contribute to the lysis of tumor cells in vivo. Accordingly, research has been directed toward enhancing both the activation state and the specificity of IL-2 induced killer cells in humans. Based on in vitro and animal data, the retransfusion of tumor-infiltrating lymphocytes has been shown to mediate the regression of metastatic neoplasms in up to 50% of patients receiving systemic IL-2. Considerable toxicity from the use of high-dose IL-2 has prompted attempts to develop low-dose regimens which allow for the outpatient treatment of patients presenting poor prognosis. While in most clinical trials involving IL-2, patient follow-up has been short, and no or only limited data have become available from controlled prospective and randomized clinical studies, IL-2 has shown some promise in patients with metastatic renal cell cancer or malignant melanoma. Novel approaches toward the improvement of clinical efficacy of IL-2 include local (e.g., intracavitary) application or combinations with other cytokines such as interferon-alpha or cytostatic drugs.