Sixty-one children aged 4-18 years with cystic fibrosis were interviewed about their understanding of the illness and their attitude towards it. Responses were compared across three age groups: 4-6 years, 7-11 years and 12-18 years, corresponding to Piagetian stages of cognitive development. While there was an obvious progression in knowledge with advancing age, the children's attitude to their illness and therapy varied remarkably between age groups. Those in the 4-6 year group were more positive about themselves and about the effect of therapy than those in the older groups. More than half knew that they were born with the illness, in contrast to published reports of healthy children's concepts of illness causation for this age group. Children in the 7-11 year group were least likely to feel better after therapy and were least likely to mention anything positive about having cystic fibrosis, despite having a higher mean Schwachman clinical score than those in the oldest group.