The number of reported Japanese cases was smaller than those in European countries and the United States. The scarcity of clinical cases may reflect underdiagnosis rather than low incidence. The reported incidence of nosocomial pneumonia is directly correlated with two factors: the ready availability of specialized diagnostic tests in-house(especially sputum culture and urinary antigen assay) and the presence of Legionella in the hospital water supply. The Guideline for Prevention of Nosocomial Pneumonia published in 1994 by the Hospital Infection Control Practiced Advisory Committee of the CDC contains two strategies. The first addressed the prevention of Legionnaires' disease in hospitals with no identified cases(primary prevention), while the second deals with prevention in hospitals with identified cases(second prevention). Many excellent reviews have been published, but this review focuses on preventing nosocomial Legionella pneumonia.