ECD is a rare, non-Langerhans form of histiocytosis. The first case was reported in 1930 by Chester and the term ECD was first described by Jaffe in 1972. Many authors have suggested that ECD is a part of the spectrum of Langerhans cell histiocytosis(LCH) , whereas others believe ECD as a distinct lipoidosis because of the symmetric distribution of the long bone lesions on the roentgenogram and the lack of Langerhans granules and S-100 antibody staining in the foamy histiocytes. We experienced a case of Erdheim-Chester disease (ECD) with periodic night fever, myalgia and both knee pain, the first case reported in Korea.