Experimental studies in animals have shown that therapy with high-dose interleukin-2 either alone or in combination with lymphokine-activated killer cells can reduce established pulmonary and hepatic metastases. Based on these experiments, recent clinical trials have shown that therapy with high-dose interleukin-2 alone or in combination with lymphokine-activated killer cells can mediate the regression of established metastatic disease in selected patients with advanced malignancy. Of 221 patients with advanced cancer treated with this immunotherapy, 16 have had a complete regression of all metastatic cancer, and an additional 26 have had a partial regression (greater than 50% reduction) of cancer. Toxicity from treatment was primarily due to increased capillary permeability, which led to fluid extravasation and organ dysfunction. Based on these findings, new approaches are being explored, including the use of tumor-infiltrating lymphocytes and combinations of lymphokines. These studies show that the regression of established growing cancer can be mediated by manipulating the immune system.