Background: Cystic fibrosis (CF) patients have chronic pancreatic insufficiency leading to malabsorption of fat-soluble vitamins, including vitamin D which can contribute to poor skeletal health and respiratory function. Objective: This study evaluated the prevalence of vitamin D insufficiency and its impact on bone and respiratory health in adults with CF. Design and measurements: This was a retrospective study in which data were collected from medical records over a 2-year period. Data included patient demographics, lung function, biochemical data, bone mineral densities, X-rays and ascertainment of use of vitamin supplements. Data were collected from medical records at a single accredited CF Center. Serum 25-hydroxyvitamin D [25(OH)D] levels and bone mineral density studies were also collected. Patients: A total of 185 adults with CF were identified with a mean age of 29 ± 9 years. Results: The prevalence of vitamin D insufficiency [25(OH)D < 75 nmol/l] was 76%. Mean serum 25(OH)D concentrations were 58.8 ± 30 nmol/l. Use of specific vitamin D supplementation was protective against vitamin D insufficiency whereas use of multivitamins was not. There was a small, but significant, positive association between serum 25(OH)D and FEV1 per cent predicted after controlling for age, gender, BMI and race (R2 = 0.30, P < 0.001). A high prevalence (27%) of vertebral fractures was detected on lateral chest X-ray. Conclusions: The prevalence of vitamin D insufficiency and poor skeletal health is high in the US CF population. Vitamin D status appears to be positively associated with lung function. Prospective studies to examine the impact of correction of vitamin D insufficiency on skeletal and lung health in adult CF are warranted. © 2008 The Authors.