The prevalence of human rhino-, entero-, and coronaviruses was investigated by RT-PCR in nasopharyngeal aspirates from 107 children without concurrent respiratory symptoms. The children were admitted to the hospital for elective surgery. The parents filled a questionnaire about the occurrence of respiratory symptoms four weeks before and two weeks after the surgery. The rate of viral detection was 45% in children with related past or recent respiratory infection whereas 20% of the samples taken from children without any related past or recent respiratory infections were positive for picornavirus RNA, P = 0.008. Thirty-one (29%) of the nasopharyngeal aspirates were positive for viral RNA, 18% for rhinovirus, and 11% for enterovirus RNA. Coronavirus RNA was not found in any of the children. Fifty-five percent of the children with virus-positive samples had an infection-related diagnosis. In addition, 81% of the children with virus-positive samples had had previously respiratory symptoms or there were concurrent respiratory symptoms in other family members. Only four of the 31 virus-positive samples were from children without infection-related diagnosis or recent past (or immediate future) respiratory symptoms.