Bone marrow biopsies have been taken in 28 patients on or approaching maintenance haemodialysis before and after treatment with monthly intravenous iron-dextran (Imferon). Stainable marrow iron was compared with blood Hb, plasma ferritin, erythrocyte protoporphyrin and MCH levels at the time of biopsy, and with previous or subsequent responses to iron in terms of blood Hb and MCH. There was a positive correlation between bone marrow iron and plasma ferritin levels both before and after iron therapy. All the patients with excess marrow iron had high plasma ferritin. However, six patients with low or normal marrow iron also had high plasma ferritin, two of these before iron therapy was given. Seven of the eight patients with no detectable marrow iron had low plasma ferritin. Three of the 28 patients failed to respond to iron with an increased blood Hb. MCH increased in all patients studied during iron therapy. All but one of the patients with high plasma ferritin prior to iron therapy responded well to iron. Although a good correlation between plasma ferritin and marrow iron can be shown in dialysed patients given intravenous iron-dextran, a high plasma ferritin level in an individual patient should not by itself preclude iron therapy.