High-dose bolus interleukin-2 (IL-2) was granted Food and Drug Administration approval for the treatment of metastatic renal cell carcinoma based on its ability to produce durable responses in a small number of patients. Results from randomized phase III trials suggest that regimens involving lower doses of IL-2 alone or in combination with interferon produce fewer tumor regressions of decreased overall quality. Because of the toxicity and limited efficacy of this treatment, recent studies have focused on identifying predictors of response (or resistance) to IL-2 therapy. This year, investigators will launch a clinical trial designed to prospectively determine whether patients who are more likely to respond to high-dose IL-2 can be identified before therapy is initiated.