We report 32 cases of culture-proven influenza A (A/Sydney) caused by virus imported into mainland US military barracks from Puerto Rico in July 1999. Despite the fact that the shelf life of the influenza vaccine is 18 months and that the outbreak strain was a component of the previous year's vaccine, no vaccine was available from manufacturers, owing to US Food and Drug Administration regulations. Formal consideration should be given to extending the date of expiration and to maintaining a supply of the influenza vaccine year-round.