A hospital-based study of acute viral infections of the respiratory tract in Thai children, with emphasis on laboratory diagnosis. The hospital-based study described here examined the viruses found in 738 children less than 5 years old who presented at Ramathibodi Hospital, Bangkok, Thailand, from January 1986 to December 1987 with acute respiratory tract infections. Three methods for detection of viral infection are compared: direct examination of epithelial cells of the respiratory tract with the use of fluorescent antibody staining, isolation of virus, and measurement of antibody in acute- and convalescent-phase sera. Viral infections were found in 44.7% of the study population. Diagnosis by the examination of epithelial cells with the fluorescent antibody staining procedure was found to have several deficiencies; however, this technique was the most sensitive for diagnosis of infection due to respiratory syncytial virus. Isolation of virus was the best method for identification of adenoviruses, parainfluenza 1 and 3 viruses, and influenza B virus. Problems associated with serodiagnosis included failure to obtain specimens of convalescent-phase blood in 24.5% of cases and insensitivity of serodiagnosis for young children except for the identification of antibody to influenza A virus. The combination of all three tests yielded the best rate of detection of virus. 