During 31 days (5 week) from December 9th 1985 to January 8th 1986, Influenza B epidemic were observed in Seoul city. Epidemic peak was shown at 52nd week in 1985. The Epidemic associated viruses were determined as B/yamanish/510/84 and B/Ann Arbor/1/86-like strain. From total of 124 specimens of influenza-like illness children virus isolation rate were average 4.0%, much lower than that of influenza A virus isolation experience. In the epidemic influenza B infection were predominantly involved in school age children.