At present, the viruses known to in volve inacute respiratory infection are influenza viruses, parainfluenza viruses, respiratory syncytial virus, adenoviruses, entero-viruses, rhinoviruses, herpes simplex virus, cytomegalovirus and coronaviruses. The same viruses can cause different disease syndromes and vice versa so that diagnosis for etiologic viruses can only be established by laboratory studies. The purposes of this study are to 1) isolate and identify the viruses associated with respiratory tract infections in pediatric patients, 2) investigate on age and seasonal incidence of those respiratory viruses. The study cases were pediatric patients of age under 13 years who visited the out-patient clinic of Siriraj Hospital and were diagnosed as acute respiratory infection. A total number of 391 throat swab specimens were collected for virus isolation throughout the study period from March 1986 to February 1987. All specimens were inoculated into 4 types of cell cultures, i.e. HEp-2, MRC-5, MDCK and LLC-MK2, and also into chick embryonated eggs. The viruses isolated throughout the study period were 9 strains of RSV (2.30%), 8 CMV (2.05%), 6 enterovirus (1.53%), 4 parainfluenza virus type 3 (1.02%), 4 adenovirus (1.02%), 4 HSV (1.02%), 3 rhinovirus (0.77%), 2 influenza virus (0.51%), 2 parainfluenza virus type 1 (0.51%) and 3 unidentified virus (0.77%). Mixed infections between RSV and HSV were found in 4 patients. These two influenza strains were isolated in chick embryoes but not in cell cultures

and they were antigenically related to A/Singapore/6/86 (H1N1) and A/Taiwan/1/86 (H1N1) which were the major strains spread in Thailand throughout the year 1986. Each respiratory virus had its importance in certain age-group. Virus isolation rate is the highest in the patients with URI of age-group 0-6 months (31.58%), and CMV was the most common virus isolated from cases under one year of age. Incidence of other viruses in certain age-group could not be seen cleary because our isolation rate was very low. However, we could get a rough idea that RSV was seen nearly in every age-group, and infection by parainfluenza virus, enterovirus and adenovirus were more common in younger age. Concerning seasonal incidence, it is found that parainfluenza virus type 3 was prevalent during March to June, rhinoviruses and CMV spread during cooler months, i.e. October to February, while RSV spread throughout the year. For other viruses, seasonal pattern was not seen because of the very low isolation rate.