The urinary antigen for Legionella was developed in the late 1970s9,10 and is one of the most successful of the rapid diagnostic microbiology tests. Unfortunately, the test only can reliably detect one species, Legionella pneumophila, and only one serogroup. Nevertheless, it has significant advantages over direct fluorescent antibody testing for Legionella, antibody serology, and culture, including its relatively low cost and rapid performance. Most importantly, the results affect management at the point of care. As a result, the urinary antigen test now is the most common method used to make the diagnosis of Legionnaire disease. A dramatic increase in the proportion of cases of Legionnaire disease diagnosed as a result of the urine antigen test has occurred worldwide. In Europe, only 15% of cases were diagnosed with use of the urine antigen test in 1995 compared with >90% in 2006.11 Among the 10,753 cases of Legionnaire disease reported to the Centers for Disease Control and Prevention between 1980 and 1998, a significant increase was seen in the proportion of patients with a positive urine test; diagnosis as a result of urine antigen testing increased from 0% to 69% over this period, and the proportion likely is higher today.