Legionella pneumophila DNA can be detected in serum from patients with Legionnaires' disease (LD). We explored this observation studying the kinetics of L. pneumophila DNA in serum samples in relation to C-reactive protein (CRP). Eleven hospitalized patients with LD were studied. Diagnosis was made by Legionella urinary antigen test in 8 patients and seroconversion in 3 patients. A macrophage infectivity potentiator (MIP) real-time PCR was performed on 31 serum samples, including 20 follow-up serum samples. Serum samples obtained on the day of admission were MIP PCR-positive in 7 (64%) and MIP PCR-negative in 4 (36%) patients. Three (75%) of the 4 patients with a MIP PCR-negative serum sample on the day of admission became positive during follow-up. Overall, L. pneumophila DNA was detected in serum samples from 10 of the 11 patients (91%). CRP levels in the 7 patients with a positive MIP PCR serum sample on day of admission (499 +/- 144 mg/l; median +/- SD) were significantly higher than those in the 4 patients with a negative MIP PCR serum sample on the day of admission (244 +/- 97 mg/l). No difference in the severity of the disease on the day of admission was found between these patients. The presence of L. pneumophila DNA in serum is a common phenomenon in hospitalized patients with LD, although in some cases it is not yet present on the day of admission. L. pneumophila DNA in serum on the day of admission correlates with high CRP levels, but not with the severity of the disease.