Background : Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract disease in infants and young children. Both the magnitude , intensity of infection and the host response to RSV infection determine the severity and intensity of disease. Objective : Our goal was to evaluate the effect of immune response (RSV IgA) and inflammatory mediators (LT-E4), in addition to the influence of host factors on the severity of the disease. Methods : This was a randomized, prospective study in two Iraqi pediatric hospitals. One hundred and twenty –three infants (mean age: 6.99±0.62 ,71 boys & 52 girls) ,with a first episode of acute bronchiolitis were randomly divided into eight treatment groups: oxygen plus intravenous fluid, montelukast pediatric chewable tablet,salbutamol syrup,salbutamol nebulizer ,combination of both oral plus nebulized salbutamol,dexamethasone IV injection, hydrocortisone IV injection, and azithromycin suspension. Control infants with non respiratory diseases were also studied for comparisons. The measured parameters was RSV IgA titer, LT-E4 titer, and a variety of environmental and host factors that may contribute to the severity of RSV bronchiolitis.Severity of bronchiolitis was based on the quantization of lowest O2 saturation and the length of hospital stay. Results : There were significant increase in RSV IgA values in patients (1.58 ± 0.24 U/mL) compare to the control (0.36 ± 0,03 U/mL);also there were a significant increase in the leukotriene E 4 values in patients (2.66 ± 0.52 ng/ml) compared to the control infants(0.15 ± 0.007 ng/ml). Age was found to be a significant factor in the severity of infection. The younger an infant was, the more severe the infection tended to be as measured by the lowest oxygen (O2) saturation. We also found that infants exposed to postnatal cigarette smoke from the mother had a lower O2 saturation than those not exposed. Although a history of maternal atopy seemed to be protective. Conclusi