This systematic review presents the prevalence of iron deficiency, iron deficiency anemia, and anemia among preschool children and nonpregnant women of reproductive age in 23 countries with a ranking of low, medium, or high in the Human Development Index. Data were pooled and the proportion of anemia attributable to iron deficiency was estimated by region, inflammation exposure, anemia prevalence, and urban/rural setting. For preschool-age children and nonpregnant women of reproductive age, the proportion of anemia associated with iron deficiency was 25 percent and 37 percent, respectively. The proportion of anemia associated with iron deficiency was lower in countries where anemia prevalence was below 40 percent, particularly in rural populations, and in countries with very high inflammation exposure. The authors note a lower proportion of anemia associated with iron deficiency than the previously assumed 50 percent in countries with a low, medium, or high Human Development Index ranking, and recommend that anemia reduction strategies and programs be based on country-specific data.