The acute lesion in Legionnaires' disease pneumonia is an acute fibrinopurulent bronchopneumonia in which the alveoli are filled with many neutrophils and macrophages and abundant fibrin. There is only slight necrosis. Although characteristic, the lesion is not specific for this agent. However, the association with this lesion of myriad small pleomorphic rods, which stain well with Dieterle's silver-impregnation method but poorly or not at all with Gram-type stains, is uncommon except in Legionnaires' disease pneumonia. Final diagnosis requires isolation of the organism or immunofluorescent studies of the tissue, sera, or both. The full spectrum of the pneumonia is not known, but organization has been reported once. No definite anatomic correlate for the extrathoracic manifestations of Legionnaires' disease has been identified nor has the organism been found at extrathoracic sites.