OBJECTIVE Durvalumab and atezolizumab are approved as first-line therapy in extensive-stage small-cell lung cancer. Although cost-effectiveness analyses compared these immunotherapy drugs with standard chemotherapy-alone regimens, no head-to-head cost-effectiveness comparisons for these treatments exist. The aim of the present analysis is to determine the cost-effectiveness of durvalumab and atezolizumab as first-line therapy for extensive-stage small-cell lung cancer from the US payers' perspective. METHODS This study is based on two placebo-controlled, phase 3 clinical trials: CASPIAN and IMpower133. A Markov model was developed to simulate the three health states: progression-free survival, progressed disease, and death in patients with extensive-stage small-cell lung cancer. Transition probabilities were estimated from the clinical trial survival curves and extended with life-time modelling. Health utilities and direct costs of adverse event treatment were included. Main outcome was the incremental cost-effectiveness ratio (ICER) using quality-adjusted life-years saved (QALYS). Sensitivity analysis was performed to assess the impact of variables on the ICER. RESULTS Durvalumab group has a cost of $187,503 with an effectiveness of 1.08 while atezolizumab has a cost of $160,219 and an effectiveness of 0.932. Durvalumab is not cost-effective compared to atezolizumab with an ICER of $165,182 QALYS, which is over the willingness-to-pay threshold of $150,000. The model was most sensitive to durvalumab cost and the cost of treating durvalumab adverse effects. CONCLUSIONS With the ICER of durvalumab treatment group being very close to $150,000, setting a higher willingness-to-pay threshold or decreasing the drug cost through contract pricing can increase the cost-effectiveness of durvalumab compared to atezolizumab.