Lipid peroxidation, as a well-known index of reactive oxygen species activity, not only in lung biochemistry, is an oxidative process associated with membrane lipid destruction. Also, the oxidative modification of nucleic acids by reactive oxygen species is of remarkable biological importance as it may contribute to malignant conversion, but its exact role in lung cancer biology is still not clear. Our study aimed to investigate the level of lipid peroxidation ex vivo in tumour tissue and lung parenchyma obtained from patients with lung cancer. Forty-two patients with lung cancer were enrolled into the study. During a surgical resection, tumour tissue and lung parenchyma were obtained and concentration of lipid peroxidation products, thiobarbituric acid-reactive substances and Schiff bases, and spontaneous generation of hydrogen peroxide, were measured. The concentration of thiobarbituric acid-reactive substances (P<0.001) in the tumour tissue was higher than that in lung parenchyma. In small cell lung cancer as well as in squamous cell carcinoma patients, a positive correlation between spontaneous generation of hydrogen peroxide in tumour tissue and clinical stage (r = 0.43; r = 0.46; respectively) was found. Our results prove enhanced lipid peroxidation in cancer tissue as compared with matched-lung parenchyma. In small cell lung cancer and squamous cell carcinoma patients, the high level of oxidative stress, expressed as a spontaneous generation of hydrogen peroxide in tumour tissue, was associated with clinical progression of tumour's stage.