OBJECTIVE: To determine the prevalence of iron deficiency and iron deficiency anemia in the US population. DESIGN: Nationally representative cross-sectional health examination survey that included venous blood measurements of iron status. MAIN OUTCOME MEASURES: Iron deficiency, defined as having an abnormal value for at least 2 of 3 laboratory tests of iron status (erythrocyte protoporphyrin, transferrin saturation, or serum ferritin); and iron deficiency anemia, defined as iron deficiency plus low hemoglobin. PARTICIPANTS: A total of 24,894 persons aged 1 year and older examined in the third National Health and Nutrition Examination Survey (1988-1994). RESULTS: Nine percent of toddlers aged 1 to 2 years and 9% to 11% of adolescent girls and women of childbearing age were iron deficient; of these, iron deficiency anemia was found in 3% and 2% to 5%, respectively. These prevalences correspond to approximately 700,000 toddlers and 7.8 million women with iron deficiency; of these, approximately 240,000 toddlers and 3.3 million women have iron deficiency anemia. Iron deficiency occurred in no more than 7% of older children or those older than 50 years, and in no more than 1% of teenage boys and young men. Among women of childbearing age, iron deficiency was more likely in those who are minority, low income, and multiparous. CONCLUSION: Iron deficiency and iron deficiency anemia are still relatively common in toddlers, adolescent girls, and women of childbearing age.