We report on 20 cases of Legionnaires' Disease (LD) in heart transplant recipients during a two-year study. The overall frequency in this setting amounts thus to 17% (20/115). In contrast, the frequency of legionellosis in postoperative cardiac patients without immunosuppression was only 4.7% (4/84). Legionellosis was diagnosed by culture and/or antibody detection in ten (20) as well as by the detection of urinary antigens in all 20 patients. Only nine (20) patients developed pneumonia, whereas five patients presented with nodular infiltrates. The remaining six patients had moderate fever with no signs of lung infection. In contrast to the majority of patients with other underlying diseases, antigen shedding lasted for long periods in most transplant patients. In high risk patients the application of conventional diagnostic methods together with regular urinary antigen testings (i.e. twice a week) may be advantageous for the early diagnosis of Legionella infection.