Kikuchi-Fujimoto disease is a benign self-limited syndrome with distinct histopathologic characteristics. Clinical symptoms include cervical lymphadenopathy with tenderness, fever of medium grade, night sweats, nausea, vomiting, and a sore throat. It is a rare disease worldwide. Diagnosis is based on histopathologic findings from an excisional lymph node biopsy. We present the case of a 37-year-old male patient with a fever of 15 days’ duration. Clinical examination revealed no other pathologic signs except for oral aphthous lesions and a fever of unknown origin. The patient’s fever did not resolve after 15 days’ follow-up, and the results of computed tomographic study of the thorax revealed several sites of mediastinal lymphadenopathy. The diagnosis of Kikuchi-Fujimoto disease (histiocytic necrotising lymphadenitis) was based on the results of pathologic examination of the biopsy specimen. The patient’s signs and symptoms resolved without antibiotic therapy. This case report of a patient with a fever of unknown origin is presented because of the rare involvement of mediastinal lymphadenopathy without cervical lymphadenopathy. Kikuchi-Fujimoto disease should be considered in the differential diagnosis of the patient who presents with fever and mediastinal lymphadenopathy.