Background/Aims: To assess dietary and nondietary determinants of serum undercarboxylated osteocalcin (ucOC) as a measure of vitamin K status. Methods: UcOC and total intact osteocalcin (iOC) concentrations were determined by specific ELISA tests in serum samples of 231 male and 320 female participants (18–81 years) of the representative, cross-sectional Bavarian Food Consumption Survey II. Determinants of ucOC were investigated by analysis of variance, Spearman’s rank correlation coefficients and logistic regression models. Results: Mean ucOC serum concentration was 2.46 ng/ml in men and 2.34 ng/ml in women. Corresponding means of the ratio of ucOC to iOC (ucOC/iOC) were 0.28 and 0.29. Concentrations of ucOC and iOC, as well as the ratio of ucOC/iOC, strongly depended on the participant’s age. UcOC was influenced by smoking status, sports activity, and the season when blood was collected. Dietary intake of the dominant vitamin K sources, green leafy vegetables and dairy products, as well as the plasma concentration of the carotenoid lutein were inversely associated with serum ucOC values. Conclusions: In studies using serum ucOC as a measure of vitamin K supply, determinants, especially age, need to be taken into account.