Although Legionnaires' disease (LD) is frequently accompanied by pleural effusion, the characteristics of pleural effusions in LD have not been well studied. Levels of adenosine deaminase (ADA) activity in pleural fluid > 40 IU/L have a high sensitivity (81-100%) and specificity (83-100%) for tuberculosis. ADA activity in pleural effusions due to LD has not been previously reported. The case of a patient with LD complicated by a pleural effusion with high ADA activity is reported. In countries where the prevalence of tuberculosis is high and pleural fluid ADA activities are frequently measured, LD should be included in the differential diagnosis of an exudative pleural effusion with high ADA activity.