One hundred and ten children were studied 9 years after each had been in hospital for croup. They were evaluated with a questionnaire, physical examination, allergy skin testing, pulmonary function tests, and a histamine inhalation challenge. Fifty-seven of them had recurrent episodes of croup, and 33 were defined as allergic. The association between allergy and recurrent croup was highly significant. Airways hyper-reactivity was found in 23 of them, and was associated with allergy and recurrent croup. The group of children with a history of recurrent croup could be distinguished from the group with one or two episodes by male predominance, onset of the disease at a younger age, familial predisposition, a significantly greater association with allergy and airways hyper-reactivity, slightly lower expiratory flow rates in pulmonary function tests, and a tendency towards the subsequent development of asthma.