Legionnaires' disease bacterium was identified as the cause of severe pneumonia in some Nottingham, England, patients in 1977. Laboratory studies were not restricted to Nottingham but included several other areas in England. The 41 cases identified were evenly divided between areas; they also accounted for about one half of all cases for the entire country. No source of infection has been identified in these sporadic cases. There was no contact between patients, and only a few had travelled abroad before their illnesses. Serologic sampling of populations in Nottingham did not reveal a large background of infection. Only 31 of 2023 sera tested had low titer antibody to the Pontiac antigen used. Guinea-pig antisera to two positive lung extracts showed an antigenic relation to the Pontiac but not to the Togus strain, suggesting strain variation.