It is not uncommon to find cases of fever of unknown origin (FUO) in which no final diagnosis is made ever after various examinations. We investigated such cases of undiagnosed FUO, with fever persisting for a long periods and responding to steroid therapy. Among 4596 patients who were hospitalized over a 3-year period from September 1991, 25 met Petersdorf's definition of FUO. Among these 25 patients, six cases were steroidresponsive undiagnosed FUO (SR-FUO). Patients with SR-FUO had the following characteristics: marked inflammatory findings and severe illness; without a definite underlying disease being found despite various examinations; no findings which indicated any known diseases such as adult-onset Still's disease, polymyalgia rheumatica, or other collagen diseases; elderly onset, at 58 to 77 years of age (mean age: 67 years); no improvement with antibiotics, antituberculous agents, or antimycotic drugs; significant improvement of symptoms and signs with steroid therapy; and a relatively good prognosis. SR-FUO, which is not caused by any known disease and is highly responsive to steroids, is included among the FUO cases which we have difficulty in diagnosing and treating.