Fourteen fatal cases from the 1977 Vermont outbreak of Legionnaires' disease have been analyzed. Serious underlying diseases were present in all patients. The only consistent lesions were in the lungs. Bronchopneumonia was present in all cases and was confluent in most. No lobe of the lung was preferentially involved and consolidation was usually bilateral. Abscesses were evident macroscopically in only two cases. Microscopically, there was an extensive alveolar infiltrate of polymorphonuclear neutrophils and macrophages. Lysis of the inflammatory cells was frequently present and was associated with an increased number of bacteria. Coagulative necrosis of lung was present in a few cases, and the possibility of a bacterial toxin must be considered. Bacteria were well stained by the Dieterle stain and appeared Gram-negative in tissue imprints from the unfixed lung.