The first two cases of Legionnaires' disease in heart transplant patients are reported. Of interest in these cases were culture of the organism from sputum, diagnosis by percutaneous lung aspiration, bronchopleural fistula formation in the first case, which was then successfully treated with multiple-tube thoracostomies, early cavitation of lesions in both cases after the start of antibiotic therapy, positive direct fluorescent antibody staining from transtracheal aspirate 42 days after starting appropriate antibiotic therapy in the first case, apparent superiority of intravenous erythromycin therapy in the first case, and survival of both patients. Our laboratory isolated and identified the organism in both cases.