Adult T-cell leukemia/lymphoma (ATL) sometimes causes opportunistic infections. A 53-year-old woman with systemic lymphadenopathies was diagnosed with ATL by inguinal lymph node biopsies and underwent oral chemotherapy. Two months later, high grade fever, lower abdominal pain and lymphadenopathy recurred. Computed tomography revealed the presence of lymphadenopathy with marked gas formation in the pelvic lesion. Blood cultures were suggestive of septic lymphadenitis by Bacteroides fragilis (BF). This represents the first demonstration of giant lymphadenitis with gas formation caused by BF in a patient with ATL. Notably, septic lymphadenitis is pivotal in the differential diagnosis of systemic lymphadenopathy in ATL.