Oren-gedoku-to is a traditional medicine used for treating inflammatory conditions and is given by prescription in Japan, People's Republic of China, and Korea. Its anti-inflammatory effect is related to the arachidonate cascade and inhibition of cyclo-oxygenase, but research on other anti-inflammatory pathways is ongoing. We report a case of fever of unknown origin in a 33-year-old woman. The possibility of infection due to human immunodeficiency virus, autoimmune, neoplastic, or other disease was examined and excluded. Oren-gedoku-to was successfully used to treat her symptoms and may thus be a suitable treatment for patients with undiagnosed fever of unknown origin.