Thirty-four infants with acute bronchiolitis and 25 age-matched healthy controls were enrolled to investigate the possible relationship between serum malondialdehyde (MDA) and selenium (Se) levels and the occurrence and severity of acute bronchiolitis in children. Serum samples were taken for serum Se and MDA measurements, and the clinical score was assessed at admission. Blood was taken again from the children with bronchiolitis at 2 mo after discharge from the hospital. Mean serum MDA levels were significantly higher in patients with acute bronchiolitis than at the postbronchiolitis stage and the controls (4.2+/-2.5 nmol/L, 1.4+/-0.8 nmol/L, and 0.7+/-0.2 nmol/L, respectively [p<0.001]). Infants with bronchiolitis had lower mean serum Se levels at the acute stage than after 2 mo (31.7+/-28.9 micro g/L versus 68.4+/-26.4 micro g/L, p<0.05, respectively); both of which were significantly lower than the control group measurements (145.0+/-21.9 micro g/L) (p<0.001). There was a negative correlation between serum MDA and Se levels in the patient group (r= -0.85, p<0.001). The age of the patient, child's immunization status, parental smoking habit, and family crowding index were not correlated with serum Se, MDA levels, or clinical score at admission. In conclusion, increased MDA levels and impaired Se status demonstrate the presence of possible relationship of these parameters with pathogenesis of acute bronchiolitis, and antioxidant supplementation with Se might be thought to supply a beneficial effect against bronchiolitis.