The etiology of bronchial asthma (BA) is not clearly understood. In recent years, a few studies have investigated the possible role of reactive oxygen species (ROS) in the etiology of BA. There are some defense mechanisms in the organism to avoid the harmful effects of ROS. Melatonin (MEL) is synthesized by the pineal gland at night and exhibits antioxidant properties. The aim of this study was to investigate serum MEL levels, erythrocyte antioxidant enzyme activities, namely superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), and the association of the respiratory function tests, namely dynamic lung volumes; the forced vital capacity (FVC/L, FVC%), the forced expiratory volume in 1 s (FEV1/L, FEV1%), and peak expiratory flow (PEF/L/s, PEF%) in 30 patients with BA and 30 age-matched healthy controls. The levels of serum MEL, the activities of erythrocyte SOD, and the values of FVC/L, FVC%, FEV1/L, and FEV1% were significantly lower in the patients with BA than in control group. The positive correlations were observed between FVC% with erythrocyte SOD and GSH-Px, FEV1/L as well as FEV1% with erythrocyte SOD, whereas negative correlation was observed between PEF/L/s with levels of serum MEL. In conclusion, these results provide some evidence for a potential role of decreased antioxidant enzymes, MEL, and respiratory function test values in BA. Toxicology and Industrial Health 2009; 25: 411-416.