This report provides reference information for blood or urine concentrations of 27 biochemical indicators of diet and nutrition measured by the Centers for Disease Control and Prevention (CDC), Division of Laboratory Sciences at the National Center for Environmental Health (NCEH/DLS). The indicators were measured in specimens from a representative sample of the noninstitutionalized civilian U.S. population during all or part of the four-year period from 1999 through 2002. These specimens were collected by CDC's National Health and Nutrition Examination Survey (NHANES), which is conducted by CDC's National Center for Health Statistics (NCHS). NHANES is a series of surveys designed to collect data on the health and nutritional status of the U.S. population. This report covers one important facet in the assessment of nutritional status of the U.S. population: biochemical measurements. Other aspects, such as anthropometric body measurements, hematologic measurements, clinical signs of nutritional deficiency or excess, and dietary intake, are not covered. For this report, a biochemical indicator means a vitamin, iron-status indicator, trace element, or other dietary indicator with potential health relevance. Although most biochemical indicators presented in this report enter the human body from foods or supplements, the body itself produces some indicators in response to dietary intake. Blood and urine concentrations reflect the amount of nutrients and dietary compounds actually in the body from all of these sources. The biochemical indicators covered in this report are: Water-soluble vitamins and related biochemical compounds; Fat-soluble vitamins and micronutrients; Iron-status indicators; Trace elements; Isoflavones and lignans (so-called phytoestrogens). The primary purpose of this report is to improve our understanding of the concentrations of biochemical indicators of diet and nutrition in the general U.S. population and in selected subpopulations. These data will help assess inadequate or excess intake and will inform analyses on the relation between biochemical indicators and health outcomes. Other potential public health uses of the information include the following: 1) Establishing and improving upon existing population reference levels that physicians, clinicians, scientists, and public health officials can use to determine whether a person or a group of people has an unusually high or low level of a biochemical indicator of diet and nutrition. 2) Determining whether the nutritional status of special population groups, such as minorities, children, women of childbearing age, or the elderly is different from that of other groups, or whether it needs improvement. 3) Tracking, over time, trends in biochemical indicator levels in the population. 4) Assessing the effectiveness of public health efforts to improve the diet and nutritional status of U.S. residents. 5) Stimulating research to perform more in-depth analyses of the NHANES data and to generate hypotheses for future nutrition and human health studies.