Introduction: Asthma onset in children is frequently associated with respiratory infections of different etiology: atypical bacteria such as Chlamydia pneumoniae (CPN) or Mycoplasma pneumoniae (MPN), and/or viruses (Respiratory syncytial virus (RSV), Rhinovirus). Recent researches showed that acute viral infection determines a structural susceptibility through inflammatory changes, and may facilitate asthma development in atopic children. However the contribution of each factor to asthma pathogenesis is still controversial. Materials and methods: A case-control study included 129 children hospitalized in the Allergy and Pulmonology wards of the Research Institute for Maternal and Child Healthcare: the first group included 84 children with persistent asthma; the second group included 45 children with bronchial obstruction of infectious etiology. Specific antibodies were assessed using immunoenzymatic assay ELISA. Specific immunoglobulin classes A and G against CPN and MPN and immunoglobulin classes M and G against RSV were evaluated. The total serum immunoglobulin-E (IgE) titres were assessed. Statistical processing of the data was performed using the software Microsoft Excel and STATISTICA 6.0. Results: The specific antibody seroconversion for the examined infections have been found in both study groups. In the first group of patients hospitalized with asthma exacerbation diagnostic titers of antibodies were detected as follows: against MPN in 8,8% of asthma cases, against CPN in 2,9% and RSV in 11,8% of cases. Antibody response to associated infections was detected for MPN+CPN in 5,9% of children; MPN+VRS in 11,8% of children; CPN+VRS in 2,9%. Estimation of these antibodies presence in the group of children with bronchial obstruction showed the presence of anti-MPN immunoglobulins in 6,6% of cases, anti-CPN immunoglobulins in 4,4% of cases and anti-VRS immunoglobulins in 8,8% of patients. No associated infections were found in this study group. Serum IgE levels were raised from the cut off value in 91,2% of the subjects from the asthma group and in 28,9% from the second group. In addition to that, the serum IgE levels in children with asthma exacerbation was 1,5 folds higher comparing with those serologically negative (916,0±236,0 IU/ml and 647,9±104,6 IU/ml respectively, p>0,05) and correlated significantly with anti- MPN immunoglobulin G (r = 0,58). Also an inverse correlation was found between the serum IgE levels and anti-RSV immunoglobulin M (r = -0,53, p<0,01). Conclusions: Infectious factors especially Mycoplasma pneumoniae have a direct impact on asthma pathogenesis, allergic sensitization and serum IgE synthesis. Respiratory syncytial virus seems to have a role in the mechanisms of bronchial obstruction onset mostly through the increase of bronchial hyperreactivity. Thus, the intensity of allergic inflammation in respiratory airways is inversely correlated with the degree of inflammation caused by RSV.