Background:Patients with cystic fibrosis are at risk for impaired vitamin K status due to fat malabsorption from pancreatic insufficiency. This study was designed to assess vitamin K status and measure the effect of vitamin K1supplementation in cystic fibrosis patients.Methods:Eighteen outpatients participated in a crossover study to determine the effect of vitamin K1(phylloquinone) supplementation. After obtaining initial data, each subject was randomly assigned to either a 4-week study treatment of 5 mg oral vitamin K1supplementation per week, or no supplementation and then crossed over to the other treatment for a second 4 week period. Plasma, serum and urine samples were collected and analyzed pre-study and at the end of each study period.Results:The mean concentration of plasma vitamin K1for the supplemented group was significantly higher than the unsupplemented group, [0.34 nmol/L and 0.21 nmol/L, respectively (p< 0.05)]. The percent of undercarboxylated osteocalcin increased on supplementation from 17% to 31%, (p< 0.005). Prothrombin induced in vitamin K absence (PIVKA-II) increased on supplementation from 5 ng/mL to 22 ng/mL, (p< 0.005). The ratio of urinary gammacarboxyglutamic acid/creatinine was similar for both study periods.Conclusions:In contrast to other studies in cystic fibrosis, this study demonstrated a need for vitamin K1supplementation. The carboxylation state of osteocalcin and PIVKA-II were the most sensitive indices of changes in vitamin K1status. Although the 5 mg vitamin K1/week dose improved these vitamin K parameters, normal levels were not achieved.