Initially discovered as a potent T cell proliferation factor, IL-2 was soon used for cancer immunotherapy, especially for metastatic melanoma and renal cell carcinoma; however, the severe side effects of IL-2 therapy, plus the negative role of IL-2 in maintaining of CD4+ CD25+ T regulatory cells (Tregs), has somewhat dampened enthusiasm for using IL-2 in immunotherapy. This opinion article discusses the possibility of combining IL-2 with certain anti-IL-2 antibodies for reducing the dose of IL-2 needed and preferentially stimulating effector T cells, but not Tregs, an approach that might provide an improved strategy for anticancer immunotherapy. Alternatively, complexes of IL-2 with other anti-IL-2 antibodies can selectively stimulate Tregs and could, therefore, be useful for treating autoimmune diseases.