Interleukin-2 (IL-2) is a soluble factor produced by T cells that stimulates growth and activity of lymphocytes and other immune cells. First noted in murine studies, the antitumor efficacy of IL-2 has been shown to induce partial and complete regression of some tumors in human clinical trials over the past decade. Although the initial clinical success of IL-2 was in combination with lymphokine-activated killer cells, IL-2 alone has subsequently been shown to be equally efficacious. Combinations of cytokines and chemotherapies with IL-2 have been generally inconclusive and disappointing with the possible exception of interferon-alpha. Toxicities of IL-2 are common and often dose limiting. Symptomatic therapy has allowed patients to tolerate somewhat higher doses, but has not addressed the underlying mechanisms of these toxicities which may involve mediators such as tumor necrosis factor, interferon-gamma, and nitric oxide. Clinical studies assessing these factors for their involvement in the antitumor effects of IL-2 as well as its toxicities may allow better understanding of IL-2, and perhaps lead to improved cancer therapies.