Sera from 252 patients with community-acquired pneumonia were examined for the presence of antibodies to 15 antigens of 7 Legionella spp. by indirect immunofluorescent antibody testing. The sera had been collected as part of the British Thoracic Society/Public Health Laboratory Service study of community-acquired pneumonia in adults. We also examined sera from 20 patients with gram-negative sepsis. Using a limited range of antigens of L. pneumophila, nine cases of legionellosis were diagnosed in the original study. However, using antigens to other Legionella spp., we identified two further cases, caused by L. micdadei and L. gormanii respectively. Twenty-six other patients had titres of 16 or 32 to one or more antigens, most commonly L. bozemanii serogroup 1, L. micdadei and L. dumoffi. None of the patients with non-legionella pneumonia, however, had significant changes in legionella antibody titres. All of the patients with Gram-negative sepsis had titres of less than 16.