Background <p>Adult T-cell leukaemia/lymphoma (ATLL) is a haematological malignancy caused by infection with human T-cell leukaemia virus type 1 (HTLV-1). ATLL arising in the oral and maxillofacial region is extremely rare. We report a rare case of asymptomatic mandibular involvement and present a brief literature review of previously reported oral ATLL cases.</p> Case presentation <p>A 72-year-old man presented with a cutaneous mass on the thigh. Whole-body ¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography (¹⁸F-FDG PET/CT), performed for systemic evaluation, revealed intense uptake in the anterior mandibular body, although no intraoral symptoms or swelling were observed. CT demonstrated trabecular bone densification, while MRI showed bone marrow signal alteration with diffusion restriction. Biopsy revealed infiltration of atypical lymphocytes. Immunohistochemistry demonstrated positivity for T-cell markers, and DNA Southern blot analysis confirmed monoclonal integration of HTLV-1, leading to a definitive diagnosis of ATLL. The patient achieved transient complete remission with chemotherapy and electron beam therapy but died 15 months after onset.</p> Discussion <p>ATLL involving the jaw is extremely rare and lacks characteristic imaging features. This is the first report of asymptomatic ATLL of the mandible detected during systemic evaluation by ¹⁸F-FDG-PET/CT. In the present case, PET/CT contributed to the detection of a clinically occult mandibular lesion, whereas CT and MRI also demonstrated abnormal findings. A review of previously reported oral ATLL cases further supports the rarity of mandibular involvement.</p> Conclusion <p>¹⁸F-FDG PET/CT is useful as an adjunctive modality for detecting latent mandibular lesions in ATLL, and biopsy is recommended when abnormal uptake is observed, even in asymptomatic cases.</p>