"Iron excretion test" was performed as an entirely new method to study iron metabolism in iron deficiency anemia. In this test, urinary iron excretion and serum iron levels were measured after intravenous administration of 10mg of saccharated iron oxide. Thirty men and 68 women were studied with this test. They were divided into four groups: 62 cases of idiopathic hypochromic anemia, 9 cases of anemia due to blood loss, 4 cases of postgastrectomy anemia and 11 healthy controls. Patient with idiopathic hypochromic anemia were divided into two subgroups according to the rate of recurrence. Cases that developed anemia at least two times despite the standarized treatment with iron preparations constituted a group difficult to control, while the other group was easy to control. In the difficult-to-control group, urinary iron excretion showed a significant increase until two hours after injection when compared with that of the other four groups. An "Iron excretion index" was calculated to estimate the tendency for urinary iron loss more clearly. The difficult-to-control group of idiopathic hypochromic anemia had significantly higher excretion indices than other groups, which frequently exceeded 1.0. These results suggest that excess iron excretion may be one of the causes of idiopathic hypochromic anemia. This type of anemia may be designated "Iron-losing anemia". And "Iron excretion test" would be useful to detect this condition easily.