A blind reading of direct immunofluorescent staining of sputum smears was positive in five of 21 patients with Legionnaires' disease and none of 47 control patients. Positive slides showed from eight organisms per smear (one slide) to 20 to 100 organisms per oil-immersion (magnification, 100) field (three slides). Two of the positive specimens were obtained through an endotracheal tube, one at bronchoscopy, and the other positive slides were from expectorated specimens. In addition, two open-lung biopsies and a bronchial lavage sediment from three confirmed cases of Legionnaires' disease were positive by direct immunofluorescent staining for Legionnaires' disease bacterium.