Intra- and interindividual variation of phylloquinone (vitamin K1) concentrations was investigated by repeat analysis of serum from 28 hemodialysis patients with different apolipoprotein E genotypes. Phylloquinone concentrations ranged from 0.1 to 9.0 nM, with a mean of 1.08 nM (standard deviation 1.90 nM). Most of the variation in serum phylloquinone concentrations was due to differences between patients; the ratio of intra- to interindividual variation was 0.17. Serum triglyceride concentrations and apoE genotype together accounted for 64% of interindividual variation; in contrast, only 6% of intraindividual variation could be attributed to differences in triglyceride concentrations. The small residual variance implies that different phylloquinone consumption levels contributed relatively little to the variation of serum concentrations ( < 36% of interindividual variance). The results suggest that individual disposition is a more important determinant of serum phylloquinone concentrations than dietary phylloquinone intake.