OBJECTIVE To study the therapeutic efficacy of an oral iron retard preparation in patients with iron deficiency anemia. METHODS In an investigator-initiated open study, 28 patients with iron deficiency anemia were treated with an iron sulfate sustained-release preparation (80 mg iron ions per tablet). The bioavailability was determined from the increase in the haemoglobin value under therapy and the measured blood loss. For the quantification of individual blood losses, a 59Fe labeling method was used and the 59Fe whole body retention in the observation period was measured with a whole body counter. This established method provides reliable data on the bioavailability of an oral iron preparation used. RESULTS In all patients the increased iron loss (average 6.6 ± 5.9 mg/d) was easily compensated by the amount of iron (average 11.7 ± 6.6 mg/d) taken with a daily dose of 80 mg of iron. During the observation period of 6-10 weeks there was a significant increase in haemoglobin (Hb) and ferritin concentration (mean increase of Hb from 10.2 ± 1.6 to 12.5 ± 1.5 g/dl and from ferritin from 9 ± 11 to 31 ± 23 μg/L). CONCLUSIONS Our results show that the iron sulfate retardant preparation used has a high bioavailability for iron. In all patients the anemia could be treated effectively. These findings seem to indicate that patients with chronic iron deficiency anemia can also receive therapeutically administered iron from the distal intestine sections.