question: BACKGROUND: Exacerbations of childhood asthma and rhinovirus infections both peak during the spring and fall, suggesting that viral infections are major contributors to seasonal asthma morbidity. OBJECTIVES: To evaluate rhinovirus infections during peak seasons in children with asthma, and to analyze relationships between viral infection and illness severity. METHODS: Fifty-eight children with asthma ages 6-8 years provided 5 consecutive weekly nasal lavage samples during September and April; symptoms, medication use, and peak flow were recorded. Rhinoviruses were identified using multiplex PCR and partial sequencing of viral genomes. RESULTS: Viruses were detected in 36-50% of the specimens, and, 72-99% of the viruses were rhinoviruses. There were 52 different strains (including 16 HRV-C) among the 169 rhinovirus isolates; no strains found in more than two collection periods, and all but two children had a respiratory infection. Virus-positive weeks were associated with greater cold and asthma severity (p<0.0001 and p=0.0002 respectively). Furthermore, virus-positive illnesses had increased duration and severity of cold and asthma symptoms, and more frequent loss of asthma control (47% vs. 22%, p=0.008). While allergen-sensitized vs. non-sensitized children had the same number of viral infections, the former had 47% more symptomatic viral illnesses (1.19 vs. 0.81 per month, p=0.03). CONCLUSIONS: Rhinovirus infections are nearly universal in children with asthma during common cold seasons, likely due to a plethora of new strains appearing each season. Illnesses associated with viruses have greater duration and severity. Finally, atopic asthmatic children experienced more frequent and severe viral-induced illnesses. CLINICAL IMPLICATIONS: The combination of viral infection and allergy increases the morbidity of respiratory illness in children with asthma.