At the age of 12, a child with cystic fibrosis developed severe small airways obstruction of unknown aetiology, in the absence of significant bronchiectasis. He remained resistant to medical treatment until, following an exacerbation of allergic bronchopulmonary aspergillosis 18 months later, he responded to high dose oral steroids. He now remains steroid-dependent, and suffering from multiple side-effects. Possible aetiology and further therapeutic strategies are discussed.