CRD summary: This review investigated the effects of oral vitamin K supplements on bone mineral density and fractures. The authors concluded that supplementation may reduce bone loss and that an effect on fractures has been shown in Japanese patients. The cautious conclusions reflect the limitations of the available evidence and appear appropriate. CRD commentary: This review addressed a well-defined question and the inclusion criteria were clear. The authors searched a range of relevant sources. It was unclear whether any language restrictions were imposed, although two potentially relevant studies published in Japanese were excluded, which could have affected the results of the review. Publication bias was assessed using standard methods and no indication of bias was found. The study selection and data extraction were performed by two or more independent reviewers, thereby reducing the risk of bias and errors during the review process. Study quality was assessed in terms of two features known to be associated with risk of bias. Relevant details of the included primary studies were presented. The studies were combined by meta-analysis, with clinical and statistical heterogeneity being assessed. The generalisability of the results is unclear as the majority of included studies were conducted in Japanese samples with osteoporosis or other risk factors. The authors' conclusions reflect the limitations of the evidence and appear appropriately cautious.