Purpose: Anaplastic thyroid cancer (ATC), an aggressive malignancy, is associated with a poor prognosis and an unmet need for effective treatment, especially for patients without BRAF mutations or NTRK or RET fusions. Lenvatinib is US Food and Drug Administration-approved for radioiodine-refractory differentiated thyroid cancer and has previously demonstrated activity in a small study of patients with ATC (n = 17). We aimed to further evaluate lenvatinib in ATC., Methods: This open-label, multicenter, international, phase II study enrolled patients with ATC, who had ≥ 1 measurable target lesion, to receive lenvatinib 24 mg once daily. The primary end points were objective response rate (ORR) by investigator assessment per RECIST v1.1 and safety. Responses were confirmed ≥ 4 weeks after the initial response. Additional end points included progression-free survival and overall survival (OS)., Results: The study was halted for futility as the minimum ORR threshold of 15% was not met upon interim analysis. The interim analysis set included the first 20 patients. The full analysis set includes all 34 enrolled and treated patients. In the full analysis set, one patient achieved a partial response (ORR, 2.9%; 95% CI, 0.1 to 15.3). More than half of the evaluable patients experienced tumor shrinkage; three patients experienced a > 30% tumor reduction. The median progression-free survival was 2.6 months (95% CI, 1.4 to 2.8); the median overall survival was 3.2 months (95% CI, 2.8 to 8.2). The most common treatment-related adverse events (AEs) were hypertension (56%), decreased appetite (29%), fatigue (29%), and stomatitis (29%). No major treatment-related bleeding events or grade 5 treatment-related AEs occurred., Conclusion: The safety profile of lenvatinib in ATC was manageable, and many AEs were attributable to the progression of ATC. The results suggest that lenvatinib monotherapy may not be an effective treatment for ATC; further investigation may be warranted., Competing Interests: Lori J. WirthConsulting or Advisory Role: Merck, Loxo, Blueprint Medicines, Eisai, Lilly, Bayer, ExelixisResearch Funding: Checkmate Pharmaceuticals, Lilly, Ayala Pharmaceuticals, EisaiExpert Testimony: Eisai Marcia S. BroseHonoraria: BayerConsulting or Advisory Role: Bayer, Eisai, Blueprint Medicines, Loxo, ExelixisResearch Funding: Bayer, Eisai, Exelixis, Blueprint Medicines, Loxo, Lilly Eric J. ShermanConsulting or Advisory Role: Cota Healthcare, Goldilocks, Eisai, Regeneron, UpToDate, Lilly, Blueprint MedicinesResearch Funding: Plexxikon, Regeneron Lisa LicitraConsulting or Advisory Role: Eisai, Boehringer Ingelheim, AstraZeneca, SOBI, Novartis, Bayer, MSD, Merck Serono, Roche, Bristol Myers Squibb, Incyte, Doxapharma, GlaxoSmithKline, Nanobiotix, Debiopharm Group, Amgen, IpsenResearch Funding: AstraZeneca, Novartis, Roche, MSD, Eisai, Merck Serono, Bristol Myers Squibb, Boehringer Ingelheim, Celgene, Exelixis, IRX Therapeutics, Medpace, Pfizer, Debiopharm GroupTravel, Accommodations, Expenses: Merck Serono, Bristol Myers Squibb, MSD, Eisai, AstraZeneca Martin SchlumbergerConsulting or Advisory Role: Eisai, Genzyme, Bayer, Exelixis/Ipsen Steven I. ShermanHonoraria: EisaiConsulting or Advisory Role: Exelixis, Loxo, LillyResearch Funding: Exelixis Bruce RobinsonLeadership: Cochlear, Mayne PharmaStock and Other Ownership Interests: Cochlear, Mayne PharmaConsulting or Advisory Role: Loxo, EisaiSpeakers' Bureau: EisaiTravel, Accommodations, Expenses: Eisai Patrice RodienConsulting or Advisory Role: Eisai EuropeResearch Funding: Pfizer Yann GodbertHonoraria: Eisai Europe, Bayer, LillyTravel, Accommodations, Expenses: Eisai Europe Christelle De La FouchardiereHonoraria: Merck Serono, RocheConsulting or Advisory Role: Lilly, Bayer, Amgen, Bristol Myers Squibb, Servier, Roche, Pierre FabreResearch Funding: RocheTravel, Accommodations, Expenses: Roche, Celgene, Amgen, Bristol Myers Squibb, ServierOther Relationship: Incyte, MSD Oncology Kate NewboldSpeakers' Bureau: Eisai Europe Christopher NuttingEmployment: Advanced Oncotherapy Soamnauth MisirEmployment: Eisai, Urogen Pharma Ana AlmonteEmployment: Eisai Maria E. CabanillasHonoraria: Loxo/LillyConsulting or Advisory Role: Loxo, IgnytaResearch Funding: Kura Oncology, Eisai, Roche/Genentech, Exelixis, MerckNo other potential conflicts of interest were reported.