Leukemia has been treated with chemotherapy for the past 40 years with only moderate success. A growing body of evidence suggests that by augmenting the immune system more effective results may be obtained. This is highlighted by T cell reinfusions resulting in durable remissions in patients with chronic myelogenous leukemia who have relapsed after an allogeneic transplant. Interleukin-2 is the primary growth factor for T lymphocytes and is a stimulator of natural killer cell activity. It has now been shown that a limited number of otherwise refractory leukemias can be effectively treated with interleukin-2. However, there remains a lack of correlation between the biologic and clinical effects of interleukin-2. The clinical activity of interleukin-2 appears to be greatest in myeloid leukemias. A variety of dose schedules and routes of administration make it difficult to determine if interleukin-2 given to patients in clinical remission is of benefit. Large randomized studies are necessary to explore the role of interleukin-2 in leukemia.