Background: Although diabetes is a frequent complication of cystic fibrosis (CF), patients’ behaviours tend not to comply with best practice recommendations. Using Leventhal’s Common-Sense Model, we address this issue by exploring patients’ representations of CF-related diabetes (CFRD) to better understand the discrepancy between patients’ expected and observed health behaviours.Methods: Semi-structured individual interviews were conducted with patients (n = 39) in six CF clinics in Quebec, Canada. These interviews were part of a larger research project on screening and management practices for CFRD.Results: Illness representations differed between two groups of interviewed patients: (1) one group had either CF without dysglycemia or CF with impaired glucose tolerance; and (2) the other group had CFRD. Both representations were internally consistent and encompassed Leventhal’s five dimensions of illness representation: illness identity, cause, timeline, consequences and control.Conclusions: Patients require specific information on CFRD. The screening phase could be a crucial time to help patients adjust their representations to fit the reality of CFRD.