BACKGROUND Outbreaks of travel-related Legionnaires' disease present a public-health challenge since rapid, sensitive, and specific diagnostic tests are not widely used and because detection of clusters of disease among travellers is difficult. We report an outbreak of Legionnaires' disease among cruise ship passengers that occurred in April, 1994, but that went unrecognised until July, 1994. METHODS After rapid diagnosis of Legionnaires' disease in three passengers by urine antigen testing, we searched for additional cases of either confirmed (laboratory evidence of infection) or probable Legionnaires' disease (pneumonia of undetermined cause). A case-control study was conducted to compare exposures and activities on the ship and in ports of call between each case-passenger and two or three matched control-passengers. Water samples from the ship, from sites on Bermuda, and from the ship's water source in New York City were cultured for legionellae and examined with PCR. RESULTS 50 passengers with Legionnaires' disease (16 confirmed, 34 probable) were identified from nine cruises embarking between April 30 and July 9, 1994. Exposure to whirlpool spas was strongly associated with disease (odds ratio 16.2, 95% Cl 2.8-351:7); risk of acquiring Legionnaires' disease increased by 64% (95% Cl 12-140) for every hour spent in the spa water. Passengers spending time around the whirlpool spas, but not in the water, were also significantly more likely to have acquired infection. Legionella pneumophila serogroup 1 was isolated only from the sand filter in the ship's whirlpool spa. This isolate matched a clinical isolate from the respiratory secretions of a case-passenger as judged by monoclonal antibody subtyping and by arbitrarily primed PCR. CONCLUSIONS This investigation shows the benefit of obtaining a recent travel history, the usefulness or urine antigen testing for rapid diagnosis of legionella infection, and the need for improved surveillance for travel-related Legionnaires' disease. New strategies for whirlpool spa maintenance and decontamination may help to minimise transmission of legionellae from these aerosol-producing devices.