Some study children may have been infected by a virus that is yet to be discovered leading to a false-negative test. It is also possible that some false-negative test results were caused by the technique used to obtain nasal secretions, although every effort was made to collect a proper sample. We may have detected more viruses if both culture and PCR testing had been used, although the sensitivity of PCR is known to exceed that of culture in most studies. For example, in one study, PCR tests consistently and substantially increased detection of multiple respiratory viruses compared with culture, and, in our laboratory, the use of PCR detected at least one respiratory virus in 53.4% of subjects compared with 38.3% who had virus detected by the use of direct immunofluorescence (7, 27) .