More than 9.4 million passengers traveled on pleasure cruises departing from North American ports in 2004, an increase of 13% since 2003 and 41% since 2001. Cruise ships typically transport closed populations of thousands of persons, often from diverse parts of the world. Travelers are at risk for becoming ill while on board, most commonly from person-to-person spread of viral gastrointestinal illnesses. Certain environmental organisms, such as Legionella spp., pose a risk to vulnerable passengers. During November 2003-May 2004, eight cases of Legionnaires disease (LD) among persons who had recently traveled on cruise ships were reported to CDC. This report describes these cases to raise clinician awareness of the potential for cruise-ship--associated LD and to emphasize the need for identification and reporting of cases to facilitate investigation.