Lungs from 224 patients, obtained at autopsy, were examined for Legionella pneumophila by fluorescent antibody (FA) staining. Of 121 patients who died with pneumonia, L pneumophila was present in eight cases (6.6%). (Two of the eight patients exhibited no important respiratory symptoms or fever, although pneumonia contributed considerably to their deaths. Preexisting underlying disease was present in all cases. Legionnaires' disease (LD), endemic in the central Ohio area, may cause up to 3.6% of the nosocomial pneumonias at the study site. Application of the local incidence of LD to the number of annual adult deaths in the United States indicates that many LD-associated deaths may occur each year. The study shows the importance of using the FA and Dieterle stains during routine pathological examination of lung tissue, especially from compromised hosts in endemic areas.