Introduction:Impaired vitamin K status in cystic fibrosis (CF)-has been considered as a newly emerged pathogenetic factor for reduced bone mineral density (BMD). Objectives:Our aim was to evaluate the effectiveness of vitamin K supplementation in managing bone formation abnormalities in children and adolescents with CF.Materials and methods:The statuses of vitamins K and D in relation to biochemical markers of bone metabolism and BMD were examined in 20 CF children receiving vitamin D supplements but not vitamin K supplements. Laboratory tests were carried out at the beginning of the study period and after 1 year of vitamin K supplementation (10 mg single oraldose/week) and the results were compared; the results were also compared with those of 25 healthy controls. Results and discussion:Ten of the CF patients had BMD z-score≤2.5 (n=5) or between -1 and -2.5 (n=5). Biochemical tests on patients before vitamin K supplementation revealed that the levels of osteoblastic activity markers,namely,bone alkaline phosphatase (BAP),serum osteocalcin (Gla-OC),serum carboxy-terminal propeptide of type I procollagen (PICP) and serum amino-terminalpropeptide of type I procollagen (PINP),were significantly reducedcompared with those of the controls. These patients had also lower 25-hydroxy-vitamin D (25(OH)D) and vitamin K serum levels,higher undercaboxylated osteocalcin (Glu-OC) and parathormone (PTH) levels and a higher calcium to creatinine ratio (Ca/Cr) than the controls. Vitamin K intake was associated with an increase in Gla-OC,PINP,PICP levels and a decrease in Glu-OC levels. PTH levels were lower after vitamin K supplementation without any difference in BMD z-scores.Conclusion:Our data indicate that vitamin K supplementation may have a beneficial role in bone health in CF children. Introduction: Impaired vitamin K status in cystic fibrosis (CF) -has been considered as a newly developed pathogenesis factor for reduced bone mineral density (BMD). Objectives: Our aim was to evaluate the effectiveness of vitamin K supplementation in managing bone formation abnormalities in children and adolescents with CF. Materials and methods: The statuses of vitamins K and D in relation to biochemical markers of bone metabolism and BMD were examined in 20 CF children receiving vitamin D supplements but not vitamin K supplements. Laboratory tests were carried out at the the beginning of the study period and after 1 year of vitamin K supplementation (10 mg single oral dose / week) and the results were compared; the results were also compared with those of 25 healthy controls. Results and Discussion: Ten of the CF patients had BMD z-score≤2.5 (n = 5) or between -1 and -2.5 (n = 5). Biochemical tests on patients before vitamin K supplementation revealed that the levels of osteoblastic activity markers , namely, bone alkaline phosphatase (BAP), serum osteocalcin (Gla-OC), serum carboxy-terminal propeptide of type I procollagen (PICP) and serum amino-terminalpropeptide of type I procollagen (PINP), were significantly reducedcompared with those of the controls. These patients had also lower 25-hydroxy-vitamin D (25 (OH) D) and vitamin K serum levels, higher undercaboxylated osteocalcin (Glu-OC) and parathormone (PTH) levels and a higher calcium to creatinine ratio (Ca / Cr) than the controls. Vitamin K intake was associated with an increase in Gla-OC, PINP, PICP levels and a decrease in Glu-OC levels. PTH levels were lower after vitamin K supplementation without any difference in BMD z-scores. Conlusion : Our data indicate that vitamin K supplementation may have a positive role in bone health in CF children.