The polymerase chain reaction (PCR) has been used to detect Legionella DNA in respiratory tract, serum, and urine samples from patients with pneumonia. In addition, a preliminary study using a guinea pig model suggested that testing of peripheral leukocytes by PCR may be more sensitive than testing of other samples. We used PCR to detect Legionella DNA in serial peripheral leukocyte (buffy coat), serum, and urine samples from a patient with pneumonia caused by Legionella dumoffii. Legionella DNA was detected in all 3 sample types when first collected. Buffy coat and urine samples remained positive up to 56 days after the onset of symptoms, whereas serum samples were positive from 10 up to 16 days after the onset of symptoms. Sequencing of PCR amplicons indicated the presence of L. dumoffii DNA in positive samples. It appears that buffy coat may be a useful sample to test for Legionella DNA, but further study is required to determine the precise sensitivity and to make comparisons with other sample types.