Legionella have a predilection for infecting immunocompromised patients, and transplant recipients have the highest risk. Legionella spp have been the most common cause of nosocomial pneumonia among transplant recipients at selected medical centers. Diagnosis is dependent on the ability of the clinical microbiology laboratory to isolate the organism by culture; therefore, the disease is easily overlooked. The mode of transmission of Legionella pneumophila is likely aspiration in transplant recipients. Clinical manifestations are similar to that of other bacterial pneumonias, although diarrhea is often prominent. The quinolone antibiotics (especially ciprofloxacin) are the antibiotics of choice because, unlike the macrolides or rifampin, they do not interact with the immunosuppressive agents used to counter rejection. Prevention of nosocomial legionellosis involves disinfection of the hospital's potable water system. Effective disinfection methods include superheat and flush or copper-silver ionization; hyperchlorination is no longer recommended. Routine culture surveillance directed at the hospital water supply for Legionella is mandatory in hospitals caring for transplant patients.