The prevalence of asthma and hyperreactivity have markedly increased in recent years. Evidences suggest that respiratory viruses frequently trigger exacerbations of asthma or hyperreactivity. This study was aimed at detecting and identifying those viruses that cause acute respiratory infections (ARI) in children with hiperreactivity or asthma, and determining their frequency. Besides, we evaluated if there is a correlation between the frequency of asthmatic crisis and viral infections. At National Institute of Respiratory Diseases Ismael Cosio Villegas, Mexico City, 79 children less than 15-year-old with hyperreactivity or asthma were studied. A nasopharyngeal swab sample was collected to detect viral antigens IA, IB, PI, AD and RSV. The viruses were isolated from cell cultures and their identification was by direct immunofluorescence. Blood samples were also taken for the detection of anti-viral antibodies.  Forty-eight percent of the samples were positive for at least one of the studied viruses and of these, 47% were positives for RSV. In 57% of the children with hyperreactivity or asthma, antibodies against one or more viruses were detected. June through September was the period with the largest number of viral infection cases, which correlated with the highest frequency of asthma crisis in the study group. The results suggest that the presence of respiratory virus is a risk factor for the exacerbation of hyperreactivity or asthma and the risk was higher in the months of June through September.