The persistence of rhinovirus and enterovirus RNAs was studied in the nasal secretions of children with acute expiratory wheezing (median age: 1.7 years). On admission, 84 samples from 161 (52%) children admitted to hospital were positive by reverse transcriptase-polymerase chain reaction (RT-PCR), which detects rhino- and enteroviruses simultaneously. Of the samples, 26 (16%) were positive for rhinovirus, 29 (18%) enterovirus and 29 (18%) nontypable rhino-enterovirus. After 2 weeks, 16 of these 84 (19%) samples were still positive. Rhinovirus RNA remained positive in 13 of 26 (50%) cases, whereas enterovirus RNA remained positive only in 1 of 29 (3%) cases (P=0.0001). Respiratory symptoms at 2 weeks or systemic glucocorticoid treatment during hospital stay were not related to the persistence of viral RNA. After 5 weeks, only one sample remained PCR-positive. Thirteen of the 79 (16%) asymptomatic control children were PCR-positive for respiratory picornavirus. Five of the 13 (38%) PCR-positive children developed respiratory symptoms in the following week. The study shows that after the onset of symptomatic respiratory infection enterovirus RNA may take 2-3 weeks and rhinovirus RNA 5-6 weeks to disappear from nasal mucus.