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Anaplastic thyroid cancer (ATC) is associated with an extremely poor prognosis and is resistant to the majority of chemotherapies. In 2015, lenvatinib was approved for treating ATC in Japan. The present study aimed to evaluate the overall survival (OS) of patients with ATC treated with lenvatinib. A total of 23 patients with a definitive histological diagnosis of ATC who were treated at Kanagawa Cancer Center (Yokohama, Kanagawa. Japan) were enrolled. Surgical treatment was possible in 10 patients (including one debulking surgery), and lenvatinib treatment was postoperatively started. The remaining 13 patients were not eligible for debulking surgery; thus, lenvatinib was promptly approved as a life-saving treatment. The therapeutic effect was determined according to the Response Evaluation Criteria In Solid Tumors criteria (ver.1.1). The patients exhibited a lenvatinib response rate of 17.4% and a disease control rate of 43.5%. However, lenvatinib was associated with a 100% incidence of treatment-related adverse events (AEs), with hypertension being the most common AE (91.3%). Additionally, dose interruptions and reductions were required due to the development of tumor fistulas or other tumor-related AEs, and 9 (39.1%) patients discontinued treatment due to grade 3 or higher AEs. The median OS time was 166 days. Overall, the present study demonstrated the effectiveness of lenvatinib against ATC, which is often chemotherapy-resistant. Successful treatment of fistulas developing due to tumor necrosis at the site of the primary lesion is crucial for improving the patient outcome. The response to lenvatinib in patients with ATC varies on a case-by-case basis and requires further investigation in future studies.