From November 1977 through mid-January 1978 the population younger than 25 years in the Union of Soviet Socialist Republics experienced a widespread epidemic of mild influenza (Russian flu) caused by an H1N1 virus similar to the virus that circulated worldwide during the early 1950s. Outbreaks of Russian flu occurred in school populations and military recruits in the United States starting in mid-January. Many other countries reported outbreaks of H1N1 virus in the winter of 1978. Predictions of influenza activity are always hazardous, but most experts believe that the Russian flu may occur again in the fall and winter of 1978. Other type A and B strains may also circulate; therefore, a trivalent vaccine containing A/USSR, A/Texas, and B/Hong Kong virus strains will be available. It is recommended that the chronically ill and those 65 years and older be the target populations for annual vaccination.