During lower respiratory tract infection, massive influx and activation of phagocytes is observed. Reactive oxygen species (ROS) released by macrophages and polymorphonuclear neutrophils (PMNs) kill microorganisms and cause damage to host tissues. One feature of this damage may be enhanced lipid peroxidation. Therefore, the aim of this study was to estimate the serum concentration of lipid peroxidation products in combination with clinical and biochemical indicators of inflammation in 32 patients with pneumonia. Serum concentration of lipid peroxides (CLP) and malondialdehyde (CMDA) was measured at Day 1, 4, 10 and 14 of observation, whilst chest radiography and routine blood analysis were performed at Day 1 and 14 during a 2 week treatment of lower airway infection. The CLP decreased during treatment (p < 0.05) from 0.059 +/- 0.024 to 0.043 +/- 0.017 (A532 nm) and the CMDA (p < 0.05) from 3.5 +/- 1.4 to 2.8 +/- 1.3 nmol.mL-1. A negative correlation between CLP and radiological regression (r = 0.49) and a drop in white blood cell count (WBC) (r = 0.39) was observed during the treatment. A positive correlation between CMDA and serum trypsin inhibitory capacity (STIC) (r = 0.47) and erythrocyte sedimentation rate (ESR) (r = 0.43) was found. Our data indicate that an enhanced process of lipid peroxidation occurs during pneumonia and that serum concentration of lipid peroxides returns to normal values quicker than the concentration of malondialdehyde during recovery. The use of antioxidants is suggested as an adjuvant treatment in patients with pneumonia.