In October 1981, an outbreak of 29 cases of community-acquired pneumonia occurred among adult residents of Johnson County, Iowa. Retrospective study revealed 12 cases (41 per cent) had laboratory evidence of Legionnaires' disease (LD). No significant differences in clinical or epidemiological features were found between LD cases and the other pneumonias in the outbreak. All LD cases received erythromycin; one case died for a case-fatality rate of 8 per cent. The outbreak's focus could not be identified.