Background: Bacterial infection of the lower respiratory tract initiates an acute inflammatory response. Regulation of the inflammatory response in bacterial pneumonia depends on a complex interaction between immune cells and inflammatory cytokines. Objectives: We investigated the initial levels of proinflammatory cytokines and acute phase reactants (APR), e.g. C-reactive protein (CRP), upon presentation of community-acquired pneumonia (CAP) in relation to clinical and laboratory indices of infection. Methods: We prospectively studied 28 consecutive patients with unilateral CAP. Tumor necrosis factor-α (TNF-α), interleukin (IL)-1β, IL-6 and IL-8 concentrations were measured by ELISA in both bronchoalveolar lavage (BAL) fluid and serum. Results: The concentrations of IL1-β and IL-6 in BAL fluid were found to be significantly higher in the involved lung than those in either the noninvolved lung (p = 0.008 and p = 0.012, respectively) or serum (p = 0.002 and p = 0.025, respectively). Serum CRP concentrations were increased compared to those in the involved and noninvolved lung in BAL fluid (p = 0.000 and p = 0.000, respectively). In serum and BAL from involved lung, IL-6 concentrations were higher in the systemic inflammatory response syndrome (SIRS) group than in the non-SIRS group (p < 0.05), whereas CRP, TNF-α , IL-1β and IL-8 concentrations showed no difference between SIRS and non-SIRS. There was no significant correlation between the acute physiology and chronic health evaluation II score and the cytokines. Conclusions: Our results indicate that the CRP level is higher in the serum than in the BAL fluid in the lung, and that IL-6 is the most important cytokine for the determination of the severity of the disease.