Patients with acute Legionnaires' disease (LD) pneumonia may have persistent chronic pulmonary changes, as shown by the histologic appearance of specimens of lung from patients who had survived and autopsy specimens from patients who died after a protracted clinical course. Acute pneumonia was not seen in these lungs, and LD organisms could not be identified by the direct fluorescent antibody technique or the Dieterle silver impregnation strain; instead, there was organizing pneumonia with various degrees of interstitial inflammation and fibrosis. The LD pneumonia may fail to resolve, and the lung parenchyma in areas of previous acute inflammation is not restored to normal in some patients.