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Purpose: The lingual lymph node (LLN) located on the internal surface of mylohyoid muscle is not currently included in oral cavity cancer surgery or conventional neck dissection. We investigated the risk factors for LLN metastasis and evaluated its oncologic significance in patients with oral tongue and floor of mouth squamous cell carcinoma., Patients and Methods: Adult patients (≥20 years) undergoing upfront surgery and LLN dissection for oral tongue and floor of mouth squamous cell carcinoma between 2009 and 2018 were retrospectively analyzed. Patients who had relapsed after previous treatment or had neoadjuvant chemotherapy or had not undergone surgery were excluded. Association between clinicopathological risk factors (age, gender, tumor differentiation, stage, lymphatic invasion, perineural invasion, vascular invasion, metastatic lymph node ratio, and extranodal extension) and LLN metastasis was evaluated using logistic regression analysis. Disease-free survival in accordance with LLN metastasis was evaluated by the Kaplan-Meier method., Results: A total of 51 patients were included, and LLN metastasis was found in 9 patients (17.6%). LLN metastasis was significantly associated with advanced nodal stage, poor tumor differentiation, and vascular invasion. Cox proportional-hazards regression models showed that LLN metastasis was associated with an 8.0-fold higher risk of mortality than the absence of LLN metastasis. Patients with LLN metastasis had significantly worse 5-year disease-free survival rate than those without metastasis (22.2% vs 85.7%; P < .001)., Conclusions: This study suggests that LLN metastasis is a poor prognostic factor in patients with oral tongue and floor of mouth squamous cell carcinoma. The sublingual space should be carefully evaluated preoperatively and intraoperatively., (Copyright © 2021 American Association of Oral and Maxillofacial Surgeons. Published by Elsevier Inc. All rights reserved.)