Legionnaires' disease can be diagnosed by culture, by direct detection of the bacterium or its products by using immunologic means, with a DNA probe, and by serologic means. Culture diagnosis is the most specific and sensitive test. Legionellae can be isolated from sputum samples by using selective techniques. Antibody detection is more suited for epidemiologic purposes than for use in individual cases. Immunofluorescent microscopy is a useful and rapid means of diagnosis. Alternative rapid and specific tests are urinary antigen detection and the use of a DNA probe. Culture must always be performed for optimal sensitivity and for epidemiologic purposes.