Legionnaires' disease is an illness with protean manifestations that are due to infection with Legionella pneumophila. It occurs both in epidemic and sporadic form and usually presents as an atypical pneumonia. Relative bradycardia, abnormal liver function test results, and a patient presenting with an atypical pneumonia should alert the clinician to the possibility of Legionella. The presence of systemic involvement, specifically neurological, gastrointestinal, and renal abnormalities, should further suggest the diagnosis. Patients may demonstrate multiple extrapulmonary manifestations of legionnaires' disease, sometimes without pneumonia. Several methods are available to aid the clinician in making the diagnosis of legionnaires' disease, and the use of all tests will increase the overall sensitivity.