Purpose.- Molecular processes can have a different impact on epidemiological data. Patients and methods. - The study covers 118 nasal aspirate samples taken on children hospitalized for acute asthma exacerbation for 2 years. Conventional techniques associated viral culture and immunofluorescence while molecular techniques used polymerase chain reaction (PCR). Results. - Virus presence was revealed with conventional techniques in 34% of the respiratory samples (40/118), while PCR study of viruses and genomes of Chlamydia pneumoniae and Mycoplasma pneumoniae allowed positive identification in 68% of the samples (80/118). The combination of both techniques allowed identification of an infectious agent in 77% of cases (91/118). More than one pathogenic agent was isolated in 23% of positive samples. Epidemiological study shows prevalence of rhinovirus (45%), then respiratory syncytial virus (28%) and enterovirus (8.5%). In children under 2 years of age, rhinovirus and respiratory syncytial virus have a close prevalence (respectively 42 and 36%), which is not the same result as in older children (respectively 66 and 27%). Moreover, PCR techniques allowed the identification of just a few Chlamydia pneumoniae and Mycoplasma pneumoniae (6/118). Conclusion. - In this study, molecular techniques of identification demonstrate a clear advantage in sensitivity compared to performances of viral cultures or immunofluorescence. The importance of rhinovirus and respiratory syncytial virus is remarkable while Chlamydia pneumoniae an Mycoplasma pneumoniae do not seem to be particularly involved.