OBJECTIVE: To analyze the relation between vitamin K status and bone mineral density (BMD) in women with Alzheimer's disease (AD). DESIGN: Cross-sectional study. SETTING: Outpatient departments of neurology and neuropsychiatry at a hospital in Japan. Participants One hundred women with AD (mean age, 79.8 y) and 100 age-matched community dwelling controls (mean age, 80.6 y). INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Patients were divided into 2 groups according to the degree of dementia: the mild AD group was composed of patients with a score in Mini-Mental State Examination (MMSE) of 16 and above (n=42); patients in the severe AD group had MMSE scores below 15 (n=58). We assessed body mass index (BMI). BMD was measured by computed x-ray densitometry. Serum concentrations of vitamin K 1 , 25-hydroxyvitamin D (25[OH]D 3 ), intact parathyroid hormone (PTH), and Glu osteocalcin (OC) were measured. RESULTS: BMI was significantly lower in women with more severe AD. Metacarpal BMD ( P <.02) and serum concentrations of vitamin K 1 ( P <.03) and 25(OH)D 3 ( P <.001) were significantly lower in the severe AD group than in the mild AD group. Serum levels of intact PTH and Glu OC in severely demented patients were higher than those with mild dementia ( P <.001). Serum PTH concentration correlated negatively with serum 25(OH)D 3 level ( r =-.241, P =.016). Serum concentration of vitamin K 1 correlated positively with that of 25(OH)D 3 ( r =.423, P <.001) and MMSE score ( r =.353, P <.001), and negatively with Glu OC ( r =-.580, P <.001). CONCLUSIONS: In female AD patients, nutritional vitamin K 1 deficiency may reduce production of fully carboxylated OC, which in turn may cause reduced BMD. Lower BMIs in more severe AD may imply the presence of general malnutrition in such a patient group.