ABSTRACT Accurate dietary assessment of vitamin K requires representative food composition data for specific geographical regions. The purpose of this study was to determine the contents of 3 different forms of vitamin K (phylloquinone [K1], 2′,3′-dihydrophylloquinone [dK], and menaquinone-4 [MK-4]) in representative grains, cereals, and baked goods, including breakfast foods, in the U.S. food supply. Samples were obtained as part of USDA's Natl. Food and Nutrient Analysis Program and analyzed by high-performance liquid chromatography (HPLC). Overall, breads, grains, and breakfast cereals were limited sources of K1 (range: nondetectable [ND] to 11.2 μg/100 g), with a wide range in dK (range: ND to 47.0 μg/100 g). In contrast, processed foods, such as fast-food breakfast sandwiches and baked goods, contain wide ranges of K1 (0.9 to 39.3 μg/100 g) and dK (ND to 72.2 μg/100 g). For any given food, K1 concentrations clustered within a narrow range, whereas dK concentrations had a wide range for a given food, suggestive of divergent use of hydrogenated oils in the manufacturing process. Low MK-4 concentrations (1.8 to 4.0 μg/100 g) were detected in meat- and cheese-containing breakfast foods and certain pie crusts. These data suggest that processed foods that contain K1-rich plant oils are a source of K1 and dK in the U.S. food supply.