Iron deficiency is the most common known form of nutritional deficiency. Its prevalence is highest among young children and women of childbearing age (particularly pregnant women). In children, iron deficiency causes developmental delays and behavioral disturbances, and in pregnant women, it increases the risk for a preterm delivery and delivering a low-birthweight baby. In the past three decades, increased iron intake among infants has resulted in a decline in childhood iron-deficiency anemia in the United States. As a consequence, the use of screening tests for anemia has become a less efficient means of detecting iron deficiency in some populations. For women of childbearing age, iron deficiency has remained prevalent. To address the changing epidemiology of iron deficiency in the United States, CDC staff in consultation with experts developed new recommendations for use by primary health-care providers to prevent, detect, and treat iron deficiency. These recommendations update the 1989 "CDC Criteria for Anemia in Children and Childbearing-Aged Women" (MMWR 1989;38(22):400-4) and are the first comprehensive CDC recommendations to prevent and control iron deficiency. CDC emphasizes sound iron nutrition for infants and young children, screening for anemia among women of childbearing age, and the importance of low-dose iron supplementation for pregnant women.