Since the initial description of Legionnaires' disease 2 years ago, a clearer picture of its clinical manifestations has emerged as a result of investigations of further epidemics and studies of laboratory-confirmed sporadic cases. Although individual clinical features are not sufficiently distinctive to distinguish Legionnaires' disease from other types of acute pneumonia, a composite can provide a sufficiently characteristic clinical profile to indicate the likelihood of this diagnosis. Such a profile includes high fever (above 39.4 degrees C); recurrent chills; relative bradycardia; early gastrointestinal symptoms (particularly diarrhea); prominent myalgias; microscopic hematuria; liver function abnormalities; toxic encephalopathy; nonproductive cough; absence of bacterial pathogens on Gram stain and culture of transtracheal aspirate; progression from patchy bronchopneumonia to lobar and multilobar consolidation; and frequently prompt and sometimes dramatic response to treatment with erythromycin.