During lower respiratory tract infection, massive influx and activation of phagocytes and release of reactive oxygen species (ROS) is observed. As a consequence, the lipid peroxidation in plasma may increase. The aim of this study was to estimate the plasma concentration of lipid peroxides in combination with a biochemical indicator of inflammation and to assess a possible correlation between infection and oxidative stress. Plasma procalcitonin (PCT) and peroxides (PEROX) concentrations were measured in 15 patients hospitalised for radiologically confirmed severe streptococcal community acquired pneumonia (CAP). As control values, PCT and PEROX were also measured in 10 healthy subjects.