Extensive epidemiologic-laboratory investigations were conducted in which the specific viral etiology was defined in 40% of acute respiratory illnesses in children and in 20% of such illnesses among adults. Included in the latter group was the "common cold" caused by coryzaviruses. Evidences were presented for simultaneous infection with more than one virus in certain cases of respiratory disease. Review of the contemporary literature made possible an appraisal of the relative importance of the individual viruses in acute respiratory illnesses and provided a rationale for future development of vaccines which may afford protection against a significant portion of the respiratory disease spectrum.