An imbalance between oxidative stress and antioxidative capacity may play an important role in the development and progression of bronchial asthma (BA) and chronic obstructive pulmonary disease (COPD). We carried out a study to assess the systemic oxidant-antioxidant status during the exacerbation and the stable period in patients with BA and COPD. A total of 33 patients, 16 with BA and 17 with COPD were included in the study. During the exacerbation and the stable periods, levels of malondialdehyde (MDA), activities of superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), glutathione reductase (GRd), and catalase (CAT) in erythrocytes and serum melatonin concentrations were investigated. Blood counts, respiratory functions, and blood gases of the patients were also performed. During an exacerbation period of BA, despite the decreases in GSH-Px, GRd and melatonin levels, MDA and CAT levels, and the white blood cell count, the percentage of eosinophils were significantly higher than in the stable period. Also, it was found that FEV1/L (where FEV1 is the forced expiratory volume in 1s), FVC/L (where FVC is forced vital capacity), PEF/L/s (where PEF is peak expiratory flow), pO(2) (where pO(2) is oxygen pressure) levels increased during the stable period in patients with BA. MDA and SOD values were higher in the exacerbation period than in the stable period although GSH-Px, GRd, melatonin, pH, and pO(2) values were lower in the exacerbation period than in the stable period. The blood counts and the respiratory function tests did not change between the exacerbation and the stable period of patients with COPD significantly. In conclusion, we observed that oxidative stress in the exacerbation period of patients with BA and COPD increased whereas the antioxidant enzymes and melatonin values reduced. The episodes of BA or COPD might be associated with elevated levels of oxidative stress. Copyright (C) 2009 John Wiley & Sons, Ltd.