On March 8, 2011, the Maine Center for Disease Control and Prevention (Maine CDC) received a laboratory report of a positive influenza specimen from an intensive-care unit patient who was an inmate at a prison (facility A). That same day, the state medical examiner notified Maine CDC of an inmate death suspected to be have been caused by influenza at another, nearby prison (facility B). On March 9, Correctional Medical Services (CMS), which provides health services to both facilities, notified Maine CDC that additional inmates and staff members from both facilities were ill with influenza-like illness (ILI). CMS reported that influenza vaccination coverage among inmates was very low (<10%), and coverage among staff members was unknown but believed to be low. Maine CDC assisted CMS and the Maine Department of Corrections (DOC) in conducting an epidemiologic investigation to gather more information about the two cases, initiate case finding, and implement control measures, which included emphasizing respiratory hygiene and cough etiquette, closing both facilities to new admissions and transfers, and offering vaccination and antiviral drugs to inmates and staff members. This report describes the public health response and highlights the importance of collaboration between public health and corrections officials to identify quickly and mitigate communicable disease outbreaks in these settings, where influenza can spread rapidly in a large and concentrated population. Correctional facilities should strongly consider implementing the following measures during each influenza season: 1) offering influenza vaccination to all inmates and staff members, 2) conducting education on respiratory etiquette, and 3) making documentation regarding the vaccination status of inmates and staff members accessible.