Following the discovery of Legionella pneumophila as the cause of an epidemic of pneumonia at an American Legion convention in Philadelphia, a group of related bacteria were recognized as additional human pathogens. This newly established bacterial genus, Legionella, includes the agents of Legionnaires' disease, Pittsburgh pneumonia, and several related infections. There are many similarities in the pathology of human infection caused by all the Legionella species. All produce a severe confluent lobular or lobar pneumonia, and abscess formation is not uncommon. A leukocytoclastic inflammatory infiltrate of neutrophils and macrophages, "septic" vasculitis of small blood vessels, coagulation necrosis, and focal septal disruption are characteristic but not diagnostic features. The inflammatory response is clearly that of a bacterial pneumonia with a necrotizing component, and does not resemble most mycoplasmal, chlamydial, or viral pneumonias. The bacteria can be demonstrated well by special stains. Acid fastness of Legionella micdadei, the cause of Pittsburgh pneumonia, is a helpful presumptive clue to diagnosis. The bacteria can be presumptively speciated in tissue by direct immunofluorescence. In addition, reliable recovery of the organisms on agar media now allows a specific diagnosis to be made. As a group, these infections are properly referred to as the Legionella pneumonias.