It has been reported that infections with Legionella pneumophila can lead to chronic inflammatory and fibrotic reactions in the human lung. To better characterize the nature of the residual abnormalities caused by this bacterium, we inoculated Syrian hamsters intratracheally with 108 serotype 1 L. pneumophila organisms and assessed histologic, functional, and biochemical changes at intervals up to 180 days. Acutely, L. pneumophila caused an intense alveolar macrophage (AM) and polymorphonuclear leukocyte (PMN) response within the lower air spaces, air-filled lungs were noncompliant, and there was an associated 25 to 50% increase the lung content of collagen and elastin after 10 days. An inflammatory response, consisting principally of AM and centered around the terminal bronchioles, was still prominent in some infected lungs after 90 and 180 days, and the severity of the inflammation was correlated with a persistent restrictive defect in the elastic behavior of the lung. However, by histologic examination, fibrosis was not prominent, and the more representative abnormality was one of mild, diffuse air-space enlargement. Frank emphysematous changes were present focally in some lungs. In addition, an irregularly distributed lymphocytic infiltrate and goblet cell metaplasia were present in the larger bronchi of infected animals. We conclude that a single infection with L. pneumophila is capable of causing long-term inflammatory reactions in the lung, with morphologic features of both fibrosis and emphysema.