The relation of anemia to hookworm infection was studied in a random sample of 1,442 children from 14 localities throughout Costa Rica. Parasitologic examinations were done by a modified Stoll technique and the Ritchie concentration method, hemoglobin determinations by the cyanomethemoglobin method, and serum protein determinations by the Biuret method and by electrophoretic fractionation. Light hookworm infection was found in 14% of these children; 2% had moderate to severe infection. Moderate to severe anemia was found in 35% of the sample; however, over 75% of the hemoglobin deficiencies occurred among children who were not infected with hookworm. Plasma-protein levels did not differ significantly between hookworm-infected and noninfected children, but total serum-protein and serum-albumin levels correlated well with the corresponding hemoglobin values. These anemias seem to be mainly of nutritional origin; we suggest that they are caused by protein malnutrition, and that in Costa Rica hookworm infection is not the main cause of anemia.