We examined the application of the polymerase chain reaction (PCR) for the diagnosis of legionellosis. Eight intratracheal aspirates were collected from a patient with pneumonia caused by Legionella pneumophila serogroup 2, and serial 10-fold dilutions of the samples were obtained. Two samples were positive for L. pneumophila by the direct fluorescent antibody (DFA) method down to a 10(-2) concentration, and one was positive down to a 10(-4) concentration. PCR was positive for all eight samples, and the sensitivity was greater than that of the DFA method. Only one of the eight samples yielded organisms in culture: the L. pneumophila serogroup 2 strain was isolated on buffered charcoal yeast extract alpha agar as an atypical white, papillate colony.