OBJECTIVES: CheckMate-214 (NCT02231749) is a phase 3 trial comparing the efficacy of nivolumab in combination with ipilimumab (N+I) to sunitinib among patients with previously untreated advanced or metastatic renal cell carcinoma (1L RCC), with intermediate or poor prognostic risk. The trial was stopped early for demonstrating overall survival (OS) benefit for patients treated with N+I. Maximum follow-up for OS was 28.2 and 26.0 months for N+I and sunitinib, respectively. For health-economic evaluation purposes OS was extrapolated to a lifetime horizon. Objective of this study was to validate the extrapolation of sunitinib OS beyond the trial’s time horizon. METHODS: Fourteen different extrapolation models were fitted to observed sunitinib OS. A literature review identified 2 studies reporting OS for 1L treatment of RCC with sunitinib. Reported absolute OS was lower than observed in CheckMate-214, thus precluding direct comparison. Conditional survival probabilities were therefore applied to CheckMate-214 data after 12 months, the estimated mean treatment time for sunitinib. As CheckMate-214 results were for a subpopulation with intermediate or poor prognostic risk, conditional probabilities from literature were reweighted by patients’ prognostic status. RESULTS: Sunitinib OS at 24 months in CheckMate-214 was 52.5%. OS estimates based on conditional survival probabilities from the literature ranged from 43.0% to 50.9% at 24 months and from 31.5% to 40.5% at 36 months. OS extrapolations for sunitinib from CheckMate-214 lay above the OS estimates based on conditional survival probabilities from literature. CONCLUSIONS: Sunitinib survival reported in the literature differed from sunitinib survival in CheckMate‑214, reducing the value of these data for validating survival extrapolations beyond the trial’s follow-up period. Conditional survival probabilities were also dissimilar between the literature and CheckMate-214. An explanation may be the availability of nivolumab as subsequent treatment in case of sunitinib treatment failure in CheckMate-214, and historic data not reflecting recent improvements in subsequent therapy.