With the purpose to study the iron and the copper metabolisms in hookworm disease, the author performed a series of experiments by infecting dog with hookworm, by intravenous injection or oral administration of iron to hookworm patient; and determined the quantitative fluctuations of serum iron and copper in each respective group. At the same time the patients with hookworm disease and those with pure hemorrhagic anemia were compared from the standpoint of the iron and the copper metabolisms. The following are the results of these studies. 1) When normal dog is infected with hookworms, the iron content in blood decreases precipitously but the copper content, showing no marked decrease at first, will decrease remarkably only when anemia becomes striking. In the patient with hemorrhagic anemia the iron content of blood decreases along with the progress of anemia while on the contrary that of copper clearly increases. 2) In the case of hookworm patient given intravenous injection of iron, the rate of iron absorption is low but no marked change can be observed in the copper content. In the case of hemorrhagic patient given intravenous injection of iron, however, the absorption of iron is marked, and the iron requirement as well as the iron consumption are high, showing a transient decrease in the copper content. 3) In the case of hookworm patient given an anthelmintic and then successive oral adminstrations of iron, naturally the iron content in blood as well as the copper content have increased rapidly and returned to the normal level. In the case where iron is given orally to hemorrhagic anemia, the iron content in blood increases but that of copper on the contrary decreases, both recovering back to the level of normal person. Looking at these cases from the standpoint of copper metabolism, both the hookworm anemia and the hemorrhagic anemia can be readily distinguished. Consequently, the pathogenic mechanism of hookworm anemia can not be explained solely by hemorrhages, Furthermore, the cause of the decrease in copper content of blood in hookworm disease like the decrease in iron content seems to be not only due to the disturbances in its absorption but mainly due to the interference with the mobilization of copper from depot caused by toxic action of hookworm.