For patients with metastatic renal cell carcinoma (RCC), the prognosis is poor. Despite the recent approval of drugs such as sorafenib, sunitinib, and temsirolimus, durable remissions of metastatic disease are rare. This is largely due to the fact that these drugs, while effective, do not result in the eradication of disease. In 1992, the FDA approved the use of high-dose interleukin-2 (IL-2) for the treatment of patients with metastatic RCC because of a small number of patients that achieved durable responses. However, IL-2 has not become a mainstay of treatment because of the expense and toxicity associated with this therapy. This review article discusses a phase II trial that investigates predictive biomarkers that might help clinicians identify the patient population with metastatic RCC that would benefit from IL-2 therapy and therefore limit patients who receive this toxic therapy to those most likely to benefit.