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Background: Viral upper respiratory infections (URIs) are considered major risk factors for acute otitis media (AOM) in young children. We studied the epidemiology and relative roles of different viruses in respiratory infections in a cohort of 329 Finnish children followed from 2 months to 2 years of age., Methods: A nasopharyngeal aspirate (NPA) was collected whenever the child had signs and/or symptoms of URI and tested for the presence of common respiratory virus antigens or infectivity/nucleic acid (only rhinoviruses). Possible repeated detections of a given virus during a 30-day period were considered to represent a single designated virus-specific episode. AOM and URI episodes were defined in a similar way., Results: At least one virus was detected in 837 (41.7%) of the 2005 NPA specimens examined. Rates of URI and virus-specific episodes showed expected seasonal variation with major peak occurrences coinciding with or preceding those of AOM. The proportions of rhinoviruses, respiratory syncytial (RS) virus, parainfluenza virus (PIV) type 3, influenza virus A and adenoviruses were 63.1, 14.7, 6.7, 6.7 and 6.2% of the total of 761 virus-specific episodes. Influenza virus B, PIV1 and PIV2 were each responsible for approximately 1% of the episodes. AOM was diagnosed in 870 URI cases (43.4%) and in 43.3% of cases associated with a virus-positive NPA. The latter figure was clearly higher (57.7%) for RS virus-positive specimens., Conclusions: The seasonal coincidence of URI and AOM demonstrated the obvious role of URI in the pathogenesis of AOM. The occurrence of rhinoviruses and RS virus in URI was strikingly more common than that of any other virus tested. Although rhinoviruses were definitely the most frequently found viruses in NPA specimens, the association of RS virus with concurrent AOM was relatively higher than that of any other virus.