An epidemiological study of respiratory tract viral infections that occured in the Athens area during the winter season 1968-1969 is presented in this Thesis.One hundred -and -one adult patients, presenting with symptoms of an acute respiratory infection syndrome, were studied.Naso -pharyngeal swabs of these patients were examined for viruses by cell cultures in monkey kidney cells and the chick embryo in the Democritos Nuclear Genter, Biology Division.A correlation of the isolated viruses with the clinical syndrome of the patients was showm by serological tests (hemadsorption inhibition test, Hemagglutination inhibition test, Neutralization test)Common bacteria were searched in the swabs by culture in proper media. The findings may be summarized as follows :