Abstract Background: The aim of this study was to determine whether pre-existing diabetes mellitus increases the risk of rejection, infection and/or death in cystic fibrosis patients undergoing bilateral sequential single-lung transplantation. Methods: A retrospective audit of 25 consecutive patients with cystic fibrosis who underwent bilateral sequential single-lung transplantation between 1 January 2003 and 31 December 2005 at a tertiary referral hospital was carried out. Results: Although 32% patients had diabetes diagnosed before lung transplantation, 92% had random blood glucose levels ≥11.1 mmol/L requiring insulin during admission. Patients with pre-existing diabetes had increased infection-related (3.9 vs 1.2, P= 0.01) and putative rejection-related (1.4 vs 0.5, P= 0.04) hospital admissions post-transplantation compared with those without diabetes pre-transplant. During the period of observation, four of eight patients with a prior diagnosis of diabetes died compared with none of 17 patients without prior diabetes (P= 0.0055). Conclusion: Almost all cystic fibrosis patients develop hyperglycaemia after lung transplantation, but patients with prior diabetes have more complication-related admissions to hospital and a higher mortality rate.