BACKGROUND: During hypoxia, reactive oxygen species cause lipid peroxidation of cell membranes, yielding oxidation products that constitute thiobarbituric acid-reacting substances (TBARS). OBJECTIVE: To see if the concentrations of TBARS in plasma would be elevated during remission in patients with bronchial asthma. DESIGN: Thiobarbituric acid-reacting substances levels were measured by a spectrophotometric assay in plasma samples collected from patients with mild persistent (n=30) and moderate persistent (n=30) in remission of bronchial asthma. RESULTS: Among patients with moderate persistent asthma, plasma TBARS levels were significantly increased following remission (P=0.0001) - 4,9 ± 0,6 µM/l. Levels of TBARS also were elevated in patients with mild persistent asthma (P=0.0001) - 4,4 ± 0,2 µM/l versus healthy patients (3,6 ± 0,2 µM/l). CONCLUSIONS: The results show a correlation between elevated plasma TBARS levels with severity of bronchial asthma. We speculate that TBARS concentrations may be useful as a biomarker for severity of injury in patients with bronchial asthma.