This paper uses the self-regulation model of illness perceptions (Leventhal et al. , 1984) to consider the implications of different ways of thinking about the causes of illness. The relationship between anxiety/depression and knowledge or denial of illness is also considered. These issues are explored using adherence to treatment in cystic fibrosis (CF) as an example. Twenty-six CF patients took part in semistructured interviews and completed a standardized anxiety and depression scale (HAD, Zigmond and Snaith, 1983). Interview data were analyzed using Interpretative Phenomenological Analysis (Chapman and Smith, 2002). HAD data were analyzed using SPSS. The respondents displayed widely differing levels of knowledge of their condition. Some deterministic comments were also reported. Findings are discussed in relation to the information that physicians might provide to patients and families in the light of increasing knowledge about genetics in society and the genotyping of individuals with genetic conditions specifically. Any important gaps in patient knowledge could usefully be discussed at transition from pediatric to adult care and issues relating to control and genetic determinism discussed with the patients individually.