Little is known about procalcitonin (PCT) levels in patients with community-acquired pneumonia (CAP) caused by Legionella pneumophila. The aim of the present study was to investigate this infection marker in patients admitted with L. pneumophila pneumonia in relation to conventional inflammatory parameters, severity of pneumonia upon admission and clinical outcome. Eighteen patients admitted with CAP caused by L. pneumophila serogroup 1 were retrospectively examined. PCT measurements were carried out during the first week of admission in addition to measurements of C-reactive protein (CRP), white blood cell (WBC) count and registration of severity of pneumonia upon admission (CURB-65 score). The mean PCT level upon admission in patients with L. pneumophila pneumonia was 13.5 ng/mL (range 0.3-55.7 ng/mL). Mean CRP level was 397 mg/L (range 167-595 mg/L) and mean WBC count 11.7 x 10(9)/L (range 4.5-20.4 x 10(9)/L). Initial high PCT levels were indicative of more severe disease as reflected by prolonged intensive care unit (ICU) stay and/or in-hospital death. Patients admitted to the ICU showed significantly higher PCT levels compared with the remaining patients [26.7 ng/mL (range 4.6-55.7 ng/mL) vs. 6.9 ng/mL (range 0.3-29.3 ng/mL); p 0.019]. There was a significant correlation between Acute Physiology and Chronic Health Evaluation-II scores upon ICU admission and initial PCT levels upon hospital admission (r = 0.86; p 0.027). Persistently increased PCT levels during treatment were indicative of unfavourable clinical outcome. Conventional inflammatory parameters (CRP and WBC) and the CURB-65 score lacked this discriminatory capacity in our study population. PCT may therefore be a valuable tool in the initial clinical assessment and follow-up of patients with L. pneumophila pneumonia.