Purpose: The administration of interleukin-2 (IL-2) may increase the frequency of peripherally circulating FOXP3-positive regulatory immune cells, thus potentially compromising this treatment option for patients with metastatic renal cell carcinoma. The impact of IL-2–based therapy on the accumulation of FOXP3-positive immune cells in the tumor microenvironment in metastatic renal cell carcinoma is unknown. Experimental Design: Baseline ( n = 58) and on-treatment ( n = 42) tumor core biopsies were prospectively obtained from patients with clear cell metastatic renal cell carcinoma before and during IL-2–based immunotherapy. Immunohistochemical expression of FOXP3 was estimated by stereological counting technique and correlated with other immune cell subsets and overall survival. Results: A significant increase in absolute intratumoral FOXP3-positive immune cells was observed comparing baseline (median 23 cells/mm 2 ; range, 0-183) and on-treatment biopsies (median, 89 cells/mm 2 ; range, 11-388; P < 0.001). The relative increase in individual patients was median 4.7-fold, range 0.3 to 230. FOXP3-positive cells were positively correlated with CD3-positive, CD4-positive, and CD8-positive tumor-infiltrating immune cells at baseline and during treatment ( P < 0.05 in all comparisons). All patients achieving high numbers (>180 cells/mm 2 ) of on-treatment FOXP3-positive intratumoral immune cells were dead within 22 months ( n = 11), whereas patients with low numbers (<180 cells/mm 2 ) of on-treatment FOXP3-positive cells ( n = 31) had a 5-year survival rate of 19% (hazard ratio, 2.2; confidence interval, 1.03-4.5; P = 0.043). All long-term survivors were characterized by low-baseline FOXP3-positive cells and a modest absolute rise in FOXP3-positive cells. Conclusion: Intratumoral FOXP3-positive regulatory immune cells significantly increased during IL-2–based immunotherapy, and high numbers of on-treatment FOXP3-positive cells were correlated with poor prognosis in patients with metastatic renal cell carcinoma.