Legionnaires' disease is a systemic infectious disease primarily involving the lungs, with multisystemic extrapulmonary manifestations. Any species of Legionella may cause legionnaires' disease in normal and compromised hosts. The clinical diagnosis of legionnaires' disease may be made on the basis of associated extrapulmonary clinical and laboratory findings. Although no single finding in legionnaires' disease is pathognomonic, the association of key extrapulmonary constitutes a typical pattern that is diagnostically characteristic. The syndromic approach based on a weighted point evaluation system described in the article gives physicians a system to arrive at a rapid presumptive clinical diagnosis of legionnaires' disease. Definitive diagnosis of legionnaires' disease is by direct fluorescent antibody testing of respiratory specimens, serological methods, Legionella urinary antigenuria, or culture.