['The purpose of the study was to know the kinetics of procalcitonin (PCT), C-reactive protein (CRP), and white blood cell (WBC) in critically ill patients with H1N1 influenza A virus pneumonia and to compare levels of these inflammatory mediators with patients with acute community-acquired bacterial pneumonia.', 'An observational study in a mixed intensive care unit (ICU) at a general university hospital was performed. All consecutive patients admitted to the ICU with a diagnosis of severe acute community-acquired pneumonia from September 2009 to December 2009 were included. Viral (H1N1 influenza A) and bacterial microbiological diagnoses were done in every patient. At admission, demographics, comorbidities, Simplified Acute Physiology Score, Sequential Organ Failure Assessment, Lung Injury Score, and Pao(2)/Fio(2) were recorded. At admission and after 24, 48, and 120 hours, WBC, CRP, and PCT levels were obtained. Finally, hospital and ICU length of stay and mortality were recorded.', 'No differences in CRP or WBC were found between H1N1-positive patients and H1N1-negative patients (patients with acute community-acquired bacterial pneumonia). Procalcitonin levels at admission were lower in H1N1-positive patients (PCT = 0.4 [0.1-6.1] ng/mL) than in the H1N1-negative patients (24.8 [13.1-34.5] ng/mL). Procalcitonin significantly decreased with time but remained lower in the H1N1-positive group at all measurements (P < .05 for all comparisons).'] Are procalcitonin levels lower in intensive care unit patients with H1N1 influenza A virus pneumonia than in those with community-acquired bacterial pneumonia . A pilot study?