Background: Cystic fibrosis (CF) is a genetic disorder with multiorgan effects. In a subgroup with pancreatic insufficiency malabsorption of the fat soluble vitamins (A, D, E, K) may occur. Vitamin D is involved in calcium homeostasis and bone mineralisation and may have extraskeletal effects. This review examines the evidence for vitamin D supplementation in CF. Objectives: To assess the effects of vitamin D supplementation on the frequency of vitamin D deficiency, respiratory outcomes and vitamin D toxicity in the CF population. Search strategy: We searched the Cochrane CF and Genetic Disorders Group Trials Register comprising references identified from comprehensive electronic database searches and handsearches of relevant journals and abstract books of conference proceedings. Most recent search: 14 March 2011. Selection criteria: Randomised and quasi-randomised controlled trials of vitamin D supplementation compared to placebo in the CF population regardless of exocrine pancreatic function. Data collection and analysis: Both authors independently assessed the 'risk of bias' of each included trial and extracted outcome data (from published trial information) for assessment of bone mineralization, growth and nutritional status, frequency of vitamin D deficiency, respiratory status, quality of life and adverse events. Main results: Three studies are included, although only data from two were available (41 adults and children with CF). One of these studies compared supplemental 800 international units (IU) vitamin D and placebo for 12 months in 30 osteopenic pancreatic insufficient adults; both groups continued 900 IU vitamin D daily. The other (abstract only) compared supplemental 1g calcium alone, 1600 IU vitamin D alone, 1600 IU vitamin D and 1g calcium and placebo in a double-blind randomised cross-over trial; only 11 children (vitamin D and placebo groups) after six-months supplementation are included; inclusion criteria, pancreatic sufficiency or disease status of participants are not defined. There were no significant differences in primary or secondary outcomes in either study. The studies are not directly comparable due to differences in supplementation, outcome reporting and possibly participant characteristics (eg severity of lung disease, growth and nutrition, pancreatic sufficiency). There were no adverse events in either study. The third study (abstract only) compared daily calcitriol (0.25 or 0.5 micrograms) with placebo in pancreatic insufficient children and young adults, only pre-intervention data were available. Authors' conclusions: There is no evidence of benefit or harm in the limited number of small-sized published trials. Adherence to relevant CF guidelines on vitamin D should be considered until further evidence is available.