Radioiron studies were performed on 51 patients at the Montreal General Hospital in order that the mechanisms of anemia might be better understood and that disturbances of iron metabolism and erythropoiesis could be better quantitated. The patients studied fell into 5 main groups a control series (10), untreated iron deficiency anemia (21), iron deficiency anemia under therapy, (8), hemolytic anemia (6), and anemia due to erythroid hypoplasia (6). Statistical analysis was made on radioiron clearance and iron turnover data and the 5 groups were compared. Patients having iron deficiency anemia demonstrated rapid clearance of radioiron from the plasma. It was seen that the more severe the anemia as measured by the level of hemoglobin, the faster was radioiron cleared from the plasma. The paradoxical finding noted by other investigators that in untreated iron deficiency anemia iron turnover may be increased was confirmed. Iron deficient patients receiving therapy had a striking increase of iron turnover. Those patients with hemolytic disorders had rapid clearance of radioiron from the plasma and increased values of iron turnover. Inference as to the degree of bone marrow compensation was obtained by noting the extent of increase of iron turnover. Patients having anemia due to erythroid marrow hypoplasia had delayed clearance of radioiron from the plasma, normal or low values for iron turnover, and incomplete incorporation of radioiron in erythropoiesis. Incorporation data are more reliable indices of erythroid marrow activity than are iron turnover data when erythroid hypoplasia exists. Radioiron data were supplemented by the medical case history in selected instances so that the clinical usefulness of radioiron data could be emphasized. It is felt that current techniques for obtaining radioiron data permit reliable data to be obtained with facility from the routine hospital isotope laboratory and thus permit potentially greater applicability of radioiron data in the study of anemia.