Technical Abstract: Vitamin K ¿dependent proteins have been implicated in the regulation of vascular calcification, a condition that is prevalent among peritoneal dialysis patients. Vitamin K status in this patient population is unknown. In a cross-sectional study of 22 peritoneal dialysis patients selected from a Canadian hemodialysis clinic, vitamin K status was determined based on two biomarkers: serum phylloquinone and percent undercarboxylated osteocalcin. The mean age of patients was 56.5 years, and 27.3 percent were male. The mean (SD) serum phylloquinone concentration was 0.8 (0.6) nmol/L, with 27.3 percent in the abnormal range for this assay. The mean percent undercarboxylated osteocalcin was 60.2 (20.5), with 100 percent of the patients in the abnormal range. There was no correlation between phylloquinone and undercarboxylated osteocalcin in these patients. These data indicate that the majority of peritoneal dialysis patients from this one hemodialysis clinic have biochemical measures consistent with subclinical vitamin K deficiency. Vitamin K deficiency may represent a potentially modifiable risk factor for vascular calcification in this population.