The cumulative incidence (i.e. lifetime prevalence) of croup and recurrent croup (RC) was investigated by questionnaire in a large group (n = 5756) of 5-8-year-old children (mean age: 6.8 +/- 0.6 years) and the risk for asthma and/or wheezing in children with croup and RC assessed. In a random sample of the children, skin prick testing with common inhalant allergens (n = 614) and spirometry (n = 305) were performed and the results were compared between children with or without croup or RC. Of the children, 15.5% had suffered from croup, while 5.0% had had RC. The cumulative incidence was higher in boys than in girls (P < 0.05). In the children with croup or RC an increased risk for wheezing, asthma, usage of anti-asthma medication, rhinitis and hay fever was found (P < 0.01). There was no difference in the prevalence of positive skin prick tests between children with and without croup or RC. Mean percentage predicted forced expiratory volume in 1 s, forced vital capacity and peak expiratory flow was not different between children with and without croup or RC. However, children who had suffered from croup (with or without wheezing) had a lower mean percentage predicted forced expiratory flow at both 50% and 75% of forced vital capacity than those without croup (P = 0.002). A family history of hay fever, chronic bronchitis and eczema was associated with the presence of croup or RC (P < 0.01), while this was hardly the case for a family history of asthma. CONCLUSIONS Croup and recurrent croup are associated with bronchial asthma. The association seems essentially based on the presence of hyperreactive airways and less on the presence of atopy, although the latter can be considered an aggravating factor.