Urine samples from 317 patients with pneumonia and from 242 patients without pneumonia were tested using a polymerase chain reaction (PCR) system for detection of the Legionella 5S rRNA gene. The results were compared with findings obtained using the established methods for diagnosis of legionellosis. Of the 317 patients with pneumonia, 58 had confirmed legionellosis, 35 had a presumptive Legionella infection, and 224 had no evidence of Legionella infection as determined by conventional methods using published criteria. The PCR was positive for 42 patients with confirmed infections, yielding a sensitivity of 72.4%. Furthermore, 16 (47%) patients with presumptive legionellosis and five (2.2%) patients without other evidence of Legionella infection had positive results. All samples from 242 patients without pneumonia were PCR-negative. When the results for all patients were considered, the specificity of the assay was > or =98.9%. The results demonstrate that the sensitivity and specificity values of urinary PCR are in the same range as those of established methods. The use of PCR in urine complements the repertoire of rapid diagnostic methods, especially for infections caused by legionellae not belonging to Legionella pneumophila serogroup 1, in which tests for detection of urinary antigen often fail.