Aims/hypothesis Many cystic fibrosis patients are vitamin D-insufficient. Cystic fibrosis-related diabetes is a major complication of cystic fibrosis. The literature suggests that vitamin D might possess certain glucose-lowering properties. We aimed to assess the relationship between vitamin D and cystic fibrosis-related glucose intolerance. Methods We enrolled 898 cystic fibrosis patients from Sweden, Norway and Denmark. Vitamin D intake was assessed using a seven-day food record. Serum 25-hydroxyvitamin D (s25OHD) and HbA 1c were measured, and an OGTT was carried out. Multiple linear and logistic regressions were used for HbA 1c and cystic fibrosis-related diabetes/OGTT result as outcome variables, respectively. Each model was controlled for country, and for known cystic fibrosis-related diabetes risk factors: age, sex, genotype, liver dysfunction, long-term corticosteroid treatment, and lung and pancreatic function. Results Degree of vitamin D insufficiency (OR 1.36; p=0.032) and s25OHD<30 nmol/l (OR 1.79; p=0.042) were significant risk factors for cystic fibrosis-related diabetes. Accordingly, HbA 1c value was positively associated with s25OHD< 30 nmol/l and<50 nmol/l, as well as with degree of vitamin D insufficiency (adjusted R 2 =20.5% and p<0.05 in all).Electronic supplementary material The online version of this article