An animal model of Legionella pneumophila pneumonia was developed to study aerosol infection, pathogenesis, and pulmonary host defense mechanisms. Guinea pigs were exposed in an inhalation facility that limited the aerosol of L pneumophila to the snout. Bronchoalveolar lavage was used to sample airspace cells, secretions, and bacteria during developing infection in 79 exposed animals and 13 uninfected controls. An influx of polymorphonuclear neutrophils followed exponential bacterial growth during the initial three days of infection and coincided with limitation of the increase in bacteria recovered. A macrophage influx occurred at three to five days. Bacteria were eliminated from the lung by 11 days after exposure. Albumin in lavage fluid peaked at two days. Most viable L pneumophila organisms were associated with alveolar macrophages, whereas most of the bacteria associated with polymorphonuclear neutrophils were nonviable. Recruited, and possibly immune, defenses appear to be required for successful resolution of legionella pneumonia.