This report concerns one specific cause of anemia, iron deficiency, and a practical procedure for its assessment in population groups. From eight high schools in selected areas of New York State, the Yorkville High School of Women’s Service Trades showed a substantial percentage of students with substandard hemoglobin levels. Further study of 720 students (95%) of this high school serving a low income group revealed concentrations of blood hemoglobin less than 120 g/l in 8.7% of the girls. This percentage compared with 1.6% below this level in girls of similar age in 7 other high schools in New York State. 31 of 79 girls with concentrations of hemoglobin less than 125 g/l were shown to be iron deficient by response to iron therapy. The administration of iron as a supplement was followed by a significant increase in hemoglobin concentration only when initial levels of serum iron were less than 500 µg/l. Data on other nutrients in serum of these girls, vitamin A, carotene, ascorbic acid, and on alkaline phosphatase as an indication of the nutritional status with respect to vitamin D, made it seem unlikely that the diets were markedly inadequate in the nutritional factors determined, other than iron. More low values for serum vitamin A and ascorbic acid were found in this high school than in one serving a high income area. The study indicates the validity of serum iron determinations as a means of detecting inadequate iron intake in large population groups and demonstrates the usefulness of a microchemical procedure in the selection of groups of persons for study of iron deficiency.