1. Marrow-iron stores were absent or reduced in twenty-three of thirty-nine patients studied within 52 months of starting maintenance haemodialysis.
2. Oral iron was given to twelve patients (group I) with absent or reduced, and to eleven patients (group II) with normal or increased marrow-iron stores.
3. A significant increase in mean haemoglobin concentration and marrow iron was observed in group I. No significant change in mean haemoglobin concentration or marrow iron occurred in group II. Mean haemoglobin concentration after treatment was significantly higher in group I than in group II.
4. The four patients who had normal or increased marrow iron and who received no oral iron all suffered a fall in haemoglobin concentration, and three of them showed a reduction in marrow iron.
5. These findings indicate that continuous oral iron therapy should be given to all patients on maintenance dialysis to correct or prevent iron deficiency.