Many physicians are unaware of the limitations of the available tests for diagnosing infections with Legionella organisms. Geographic differences in the importance of nonpneumophila Legionella species as pathogens are underrecognized, in part because available diagnostic tests are biased toward the detection of pneumophila serogroup 1. Routine laboratory practices reduce the likelihood of culturing Legionella species from clinical isolates. Failure of seroconversion is common, particularly with nonpneumophila species; even when seroconversion occurs, it may take much longer than 4 weeks. Urinary antigen testing has insufficient sensitivity to affect clinical management in most regions of the United States, as it can reliably detect only L. pneumophila serogroup 1 infections. Polymerase chain reaction-based techniques offer hope of providing highly sensitive, rapid diagnostic tests for all Legionella species, but limitations in the current tests will make validating them difficult.