Background: KEYNOTE-564 demonstrated that adjuvant pembrolizumab after nephrectomy for high-risk clear cell renal cell carcinoma (RCC) decreased the risk of disease progression and – albeit with immature follow-up (2 years) – found a signal of improved overall survival as well. Herein, we used extrapolations from a Markov-model, accounting for the costs and toxicities of pembrolizumab relative to its efficacy, to investigate its utility at a population level. Methods: A decision-analytic Markov Model was used to conduct a cost-utility analysis of adjuvant pembrolizumab versus placebo after nephrectomy for high risk RCC, using data from KEYNOTE-564 to inform model probabilities. Transition probabilities not found in KEYNOTE-564, in addition to utility values, were extracted from the literature. Base case analyses were conducted with 5-year and 15-year time horizons (using extrapolations from 2-year outcomes), and 3-week time cycles. Primary outcomes were Quality-adjusted life years (QALYs), 2021 US Medicare costs, and Incremental cost-effectiveness ratios (ICERs). The willingness-to-pay threshold was set at $100,000/QALY. One-way sensitivity analyses were used to identify cost-effectiveness thresholds of individual parameters while probabilistic sensitivity analyses with 100,000 Monte-Carlo simulations were used to simultaneously vary all model inputs. Results: Pembrolizumab was associated with higher QALYs and costs relative to placebo (Table). Pembrolizumab was not cost-effective at a 5-year time horizon but was cost-effective at 15 years and beyond. One-way sensitivity analysis noted that pembrolizumab became cost-effective at 5-years if: 1) its administration cost was less than $5,064 (base = $10,278) or 2) if the 5-year risk of progression was 18.8% higher in placebo vs pembrolizumab (base = 9%). Using pembrolizumab’s reported hazard ratio of 0.68 for progression from KEYNOTE-564, we estimated that pembrolizumab would be cost-effective at 5-years for all patients with a Mayo Progression Free Survival Score of 10 or higher. Probabilistic sensitivity analysis found that pembrolizumab was cost-effective for 29% and 58% of microsimulations at 5 and 15 years, respectively. Conclusions: At current prices, adjuvant pembrolizumab was not cost-effective for all trial patients at a population level at 5-years after treatment but may be cost-effective for time horizons over 15 years. Instead, adjuvant pembrolizumab was found to be cost-effective only for the highest risk subsets of RCC at 5-years. Longer term trial data of progression-free survival and overall survival are necessary to confirm these extrapolations.