BACKGROUND: Rhinovirus is the most common virus causing respiratory tract illnesses in children. Rhinoviruses are classified into species A, B and C. We examined the associations between different rhinovirus species and respiratory illness severity. METHODS: This is a retrospective observational cohort study on confirmed rhinovirus infections in 134 children 3-23 months of age, who were enrolled in 2 prospective studies on bronchiolitis and acute otitis media, respectively, conducted simultaneously in Turku University Hospital, Turku, Finland, between September 2007 and December 2008. RESULTS: Rhinovirus C is the most prevalent species in our study, and it was associated with severe wheezing and febrile illness. We also noted that history of atopic eczema was associated with wheezing. CONCLUSIONS: Our understanding of rhinovirus C as the most pathogenic rhinovirus species was fortified. Existing research supports the idea that atopic characteristics are associated with the severity of the rhinovirus C-induced illness.