Legionnaires' disease is a community-acquired or hospital-acquired pneumonia, and the immunocompromised patient is at particular risk. We report a case of Legionella pneumophila serogroup 1 pneumonia in a renal transplant patient shortly after grafting. No source of infection was identified in the hospital unit, but an extended investigation located patient exposure to a shower during a weekend home stay. Sampling at the hospital, in the patient's flat, other flats, and the laundry of the same building returned only one positive result from the patient's showerhead. Strain identity was confirmed by pulsed-field electrophoresis and amplified fragment length polymorphism. Guidelines recommend Legionella-free water for transplant units, but further epidemiologic evidence is required before extending this preventive approach to the patient's home.