This is a Phase 1/2 open-label dose-escalation study of rIL-21 given in combination with sorafenib to patients with metastatic RCC. The Phase 1 part of this study will estimate the maximum tolerated dose of rIL-21 given for 1 treatment course (consisting of two 5-day cycles of rIL-21) in combination with a standard dose of sorafenib administered during a 6-week treatment course. Increasing doses of rIL-21 will be studied sequentially in different groups of patients. The Phase 2 part of the study will further evaluate the safety and preliminary antitumor activity of rIL-21 at the dose recommended from Phase 1 in combination with sorafenib. Patients will be evaluated for safety over the course of the study. Disease evaluation (tumor restaging) will be performed during the last week of each 6-week treatment course. Patients with stable disease or better at this evaluation may go on to receive additional treatment courses. Patients may be in the study for 2 to 7 months.