OBJECTIVE: The pathogenesis of bronchiolitis has not been fully identified. Immune function abnormality following virus infection may be associated with the pathogenesis. CD40 and CD40 ligand (CD40L) is a pair of co-stimulatory molecules in immunoreaction. They might play an important role in the development of bronchiolitis. This study aimed to investigate the expression of CD40 and CD40L in peripheral blood mononuclear cells (PBMCs) in children with bronchiolitis and explore their possible roles in the disease. METHODS: Thirty children with bronchiolitis, 26 children with bronchopneumonia and 30 healthy children (control) were enrolled. Flow cytometry was used to detect CD40 and CD40L expression in PBMCs. Total serum IgE level was measured using ELISA. RESULTS: Compared with the control group, CD40L expression significantly increased in the bronchiolitis and bronchopneumonia groups (P<0.05). The CD40L expression in the bronchiolitis group was significantly higher than that in the bronchopneumonia group (P<0.05). A significantly increased CD40 expression was also found in the bronchiolitis group when compared with the bronchopneumonia and the control group (P<0.01). Total serum IgE level in the bronchiolitis group was significantly higher than the bronchopneumonia and the control groups (P<0.01). CD40 and CD40L expression was positively correlated with serum IgE level in the bronchiolitis group (r=0.607, r=0.819, respectively; P<0.01). CONCLUSIONS: CD40 and CD40L expression in PBMCs and serum IgE level increased and there is a positive correlation between CD40 and CD40L expression and serum IgE level in children with bronchiolitis. Over-expression of CD40 and CD40L may play an important role in the development of bronchiolitis.［Chin J Contemp Pediatr, 2009, 11 (5):354-356］