The current histopathological aspects of legionnaires' disease, a newly discovered acute feverish disease accompanied by severe rapidly progressive pneumonias with 20% mortality are reviewed. Epidemic and sporadic cases of legionnaires' disease occurred both in the United States and many countries of Europe. The gross and microscopic pathology of autopsy specimens from patients with legionnaires' disease and pathological findings in guinea pigs inoculated intraperitoneally with Legionella pneumophilla are discussed. Some methods for isolation of specimens for laboratory diagnosis in severe pneumonias suspect for legionnaires' disease are recommended.