Objectives Impaired glucose tolerance (IGT), type 1 (T1DM) and type 2 diabetes mellitus (T2DM) have deleterious effects on cognitive performance. Hence, patients with cystic fibrosis related diabetes (CFRD) are hypothesized to show similar impairment. This study aimed to elucidate the nature and severity of any cognitive impairment in patients with CFRD compared to non-diabetic patients with CF and healthy controls matched as closely as possible for age, gender and education level. Patients with CF were also matched as closely as possible on CFTR genotype. Methods Adult (>16 years old), pancreatic insufficient patients registered to the Leeds CF Unit who had adequate verbal and written English were eligible. To date, 45 patients with insulin-treated CFRD and 47 non-diabetics who had received a normal oral glucose tolerance test (OGTT) within the past 12 months were included. 10 healthy controls were recruited to date from relatives of patients and the general population. Cognitive performance was assessed using the Cambridge Neuropsychological Test Automated Battery (CANTAB). Results Preliminary findings show that matched controls perform better than both groups of patients with CF on tests of visual memory and learning, verbal memory, visual sustained attention and executive function. Patients with CFRD performed worse than controls on tests of mental flexibility and visual sustained attention, which is consistent with the pattern of impairment shown in T1DM, and on verbal memory, which is consistent with the pattern of impairment shown in T2DM. Conclusion Like T1DM and T2DM, CFRD may have a negative impact on cognitive performance and needs to be explored.