We evaluated the potential predictive role of cytokines in patients treated with ipilimumab...Patients treated with immunochemotherapy with a serum IL-2 concentration above cut-off at baseline, had a median OS of 30.5 m while those with lower levels had a median OS of 8m (p = 0.015)...Baseline increased IL-2 levels predicted sensitivity to ipilimumab...Blood-based biomarkers are convenient for patients, and our results support prospective validation of cytokines as predictive biomarkers for ipilimumab in SCLC.