Background Vitamin K deficiency is highly prevalent in patients on dialysis and may contribute to their low bone mineral density (BMD) and increased risk of fracture. This study investigated the effect of menaquinone-7 (MK-7) supplementation on BMD in patients on chronic dialysis. Methods In a multicentre, double-blind, placebo-controlled intervention trial, 123 patients on chronic dialysis were randomised to a daily oral supplement of either MK-7 360 mu g or placebo for 2 years. BMD of the distal radius (1/3, mid, ultradistal and total), femoral neck, lumbar spine (L1-L4) and whole body was assessed by dual-energy X-ray absorptiometry. Serum levels of vitamin K1 and MK-7 and plasma levels of total osteocalcin, dephosphorylated-uncarboxylated matrix Gla protein and protein induced by vitamin K absence II were measured to assess vitamin K status. Results After 2 years, an accelerated BMD loss of the 1/3 distal radius was found with MK-7 supplementation {mean difference of changes relative to placebo -0.023 g/cm(2) [95% confidence interval (CI) -0.039 to -0.008]}, whereas the decrease in lumbar spine BMD seen in the placebo group was prevented [mean difference of changes between groups 0.050 g/cm(2) (95% CI 0.015-0.085)]. No significant effects were observed at the remaining skeletal sites. Vitamin K status strongly improved in MK-7-supplemented participants. Conclusion Compared with placebo, an accelerated BMD loss of the 1/3 distal radius was found after 2 years of MK-7 supplementation, whereas a decline in lumbar spine BMD was prevented. As such, MK-7 supplementation might modify BMD site-specifically in patients on dialysis. In aggregate, our findings do not support MK-7 supplementation to preserve bone in patients on dialysis.