We have investigated the prevalence of atopy within families of cystic fibrosis (CF) patients and compared its frequence in CF patients, their parents and their non-CF siblings. By studying families with two CF patients it was also possible to evaluate the factors influencing the development of atopy in one CF patient relative to the other patient within the same family. A significant correlation with age (P < 0.001) for skin test positivity to common allergens within CF sibling pairs was observed. In 14/18 families studied, only the older CF patient was atopic. The presence of atopy in CF patients was independent of the atopic status of their parents.