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An outbreak of influenzalike illness occurred at Marquette University, Milwaukee, Wisconsin, in February 1978. Epidemiologic and laboratory data indicate that the outbreak was due to influenza A/USSR/77 (N1H1) virus. A self-administered questionnaire inquiring about the presence and spectrum of illness was given to faculty members, dental students, and residents of an undergraduate dormitory. Age-specific attack rates were 61.5% for persons less than or equal to 22 years of age, 24.7% for those 23 or 24 years of age, and 9.7% for those greater than or equal to 25 years of age. This pattern of age-specific attack rates paralleled the age distribution of persons without hemagglutination-inhibiting antibody to influenza A/USSR/77 virus found in independent serosurveys. Prior swine influenza (H1N1) immunization provided no protection from illness in this outbreak. Past epidemic antibody titers in undergraduates were only weakly associated with clinical illness. The data on disease impact gathered in this study indicate that in a university setting influenza A/USSR/77 virus produced a short-lived outbreak of respiratory illness with a very high attack rate in young adults.