Data on sporadic cases of Legionnaires' disease in Great Britain reported to the Communicable Disease Surveillance Centre, London, and the Communicable Diseases (Scotland) Unit, Glasgow, were analysed. Eighty-four cases, including 18 patients who died, were identified with dates of onset between 1 January 1976 and 30 September 1978. The age and sex distribution showed a predominance of middle-aged men. The incidence trends suggest that true seasonal variation may occur. Eighteen patients developed illness during or shortly after a holiday abroad. All patients had pneumonia, but many also had features suggesting involvement of the central nervous system and gastrointestinal tract. Person-to-person spread was not observed.