A patient with a clinical course consistent with Legionnaires' disease rapidly developed acute respiratory failure. Sputa and bronchoscopic washings revealed many polymorphonuclear leukocytes but no organisms on gram stain and culture. The diagnosis of Legionnaires' disease was strengthened when the transthoracic aspirate revealed weakly staining gram-negative bacilli with no growth noted on culture. Serologic titers confirmed the diagnosis of Legionnaires' disease. This patient's course was complicated by cavitation which responded to medical therapy with erythromycin. This is the second reported case of Legionnaires' lung abscess.