4591 Background: IL-2 is potentially curative in a subset of patients with mRCC. We assessed our 10 years experience of outpatient subcutaneous IL-2 based immunotherapy at a single institution. Methods: Between 1999-2008, 422 consecutive patients with progressive, non-resectable mRCC were treated with first-line s.c IL-2 and s.c. interferon-alpha (IFN) based immunotherapy. Treatment was given for a maximal duration of 9 months followed by surgery of residual disease, if applicable, and then followed by observation. Patients obtaining no evidence of disease (NED) were characterized in regards to well known prognostic features and correlated to the non-NED cohort. TKIs became available at our institution in 2006 and were given following relapse. Results: Overall response rate was 15%, which included 17 complete responses and 47 partial responses. Forty patients had surgical resection of residual disease following immunotherapy. Of these, 35 patients had vital tumor cells within resected residual lesions. A total of 36 patients (9%) obtained NED with a median follow-up of 76 months (range 10.2–131 mo). The median overall survival was 15.7 months (95%CI, 13.3-18.0 mo), the estimated 5-year survival rate was 15%, and the estimated 10-year survival rate was 10%. No patients obtaining NED had received TKI at any time. Patients obtaining NED (N=36) had the primary kidney tumor in place (28%), lymph nodes mets (36%), liver mets (14%), bone mets (17%), adrenal gland mets (14%) and lung mets only (19%). A total of 25 patients (69%) had 1 or 2 disease sites. 83% had baseline Karnofsky performance of 100 or 90. 94% had MSKCC favorable or intermediate prognosis. A detailed analysis of NED-patients compared with non-NED patients will be presented. Conclusions: IL-2 based immunotherapy, either alone or in combination with surgery, resulted in no evidence of disease in 9% of patients and an estimated 10-year survival rate of 10%. IL-2 based immunotherapy should still be a viable first line treatment option as this approach is potentially curative in selected patients with mRCC. Author Disclosure Employment or Leadership Position Consultant or Advisory Role Stock Ownership Honoraria Research Funding Expert Testimony Other Remuneration Novartis