Interleukin-2 (IL-2) is able to induce the regression of metastatic cancers when administered in vivo. IL-2-activated natural killer cells and lymphocytes show, in vitro, activities against leukemic cells. To assess if in vitro observations could have significant clinical relevance, we evaluated the in vivo activity of high-dose recombinant IL-2 (6 to 8 x 10(6) IU/m2/8H intravenous bolus for 5 days) in 10 patients with acute myeloid leukemias (AML) in relapse after chemotherapy (n = 7) or autologous bone marrow transplantation (n = 3). Two patients achieved a complete remission and one had a minimal improvement in his marrow blast cells. Response was observed after one cycle of IL-2 in the two patients achieving a complete remission. These two patients relapsed at 3 and 4 months. These results showing clinical activity of high-dose recombinant IL-2 in AML invite further evaluation of this new form of immunotherapy in other clinical situations, like an adjuvant setting for selected groups of high-risk patients.