An international workshop to review the epidemiology, diagnosis, treatment, and prevention of respiratory viral infections was held in Turku, Finland, in May 1991. This workshop emphasized the following points. (1) In epidemiological studies of influenza virus, serological, clinical, and gene-sequencing methods have been used to produce a full picture of genetic evolution. Less complete information exists about other viruses, although new data on respiratory syncytial virus are emerging. (2) Tools for the diagnosis of respiratory viral infections have been developed in conjunction with the use of solid-phase immunoassays. A role remains for tissue culture in surveillance and epidemiological studies. Detection of bacterial involvement in respiratory infections has been more difficult. (3) Treatment of infections due to respiratory viruses has advanced with the use of amantadine and aerosolized ribavirin. On the other hand, many viruses remain refractory to treatment. Means for preventing influenza are established, but barriers to the development of other viral vaccines--including the existence of multiple serotypes, imperfect natural immunity, and paradoxical hypersensitivity--have proven difficult to surmount.