A simple method was devised for determination of the storage iron decrease rate (SID) and the number of days to a recurrent state of iron deficiency. Several patients with iron deficiency anemia were given intravenous doses of iron, and their serum ferritin levels were periodically assayed. A serum ferritin decrease curve was plotted semi-logarithmically; back-extrapolation of that curve yielded the period of days (D) from the starting day of iron supplement therapy to the day by which serum ferritin had decreased to 12 ug/l. The amount of administered iron that was initially stored and eventually lost during period D was calculated by subtracting the amount of iron utilized for hemoglobin increase (R) from the total amount of iron administered (T) to each patient. R was calculated from values for patient body weight and change in hemoglobin concentration prior to and after therapy. SID values were obtained from the following formula: SID = (T-R)/mg/day. SID rates and the number of days to a state of recurrent iron deficiency were measured in 12 patients. SID rates ranged from 0.8 to 9.8 mg/day, and were larger in those patients who exhibited heavier blood loss. A correlation was observed between SID values and the half-times for serum ferritin decrease, as expressed by the equation, Y = 248.5X0.129 (r = 0.129). SID values reflected the negative iron balance of each patient. That observation suggested it should be possible to select a more appropriate intravenous iron does for individual patients if their SID values are taken into full account.