A large outbreak of influenza-like and diarrhoeal illness took place over a period of 21 days in April 1984 on board a ship cruising to ports in southern Europe and northern Africa. A cohort study of the 418 passengers was made by postal questionnaire and personal interview. Of the 391 passengers who were interviewed or who returned a questionnaire, 335 (86%) were affected. Of the ill passengers, 295 (88%) had an influenza-like illness. These included 20 with signs of lower respiratory tract infection. In 24 passengers, a viral infection was diagnosed. Influenza B virus infection was identified in 14 cases; other diagnoses were influenza A, para-influenza, respiratory syncytial virus and Epstein-Barr virus infections. In two of the 81 patients tested for Legionella antibodies, a titre of 128 was found; in 16 and 44 patients, titres of 64 and 32 respectively. The outbreak was thus evidently caused by multiple pathogens mainly affecting the respiratory tract. Although most of the passengers acquired their infections on board the ship, a common source was not discovered. A steep rise in the epidemic curve the day after the air-conditioning was switched on, however, is worth noting. If and when similar instances of the 'Sick Boat Syndrome' recur, a search for environmental sources of infection is to be recommended.