Incidence rates of acute asthma attacks during weeks before and after influenza vaccination for 3 influenza seasons. The period from October 1 through April 30 was defined as the influenza season. Data were available from 3 health maintenance organizations (HMOs) for the 1993-1994 and 1994-1995 influenza seasons and from all 4 health maintenance organizations for the 1995-1996 influenza season. Access to pharmacy prescription data from the fourth health maintenance organization for 1993-1994 and 1994-1995 was unavailable.