Forty-four patients with chronic renal failure on haemodialysis for four months to eight years were studied. All recieved intravenous iron dextran 100 mg on alternate weeks. Serum ferritin concentrations correlated well with body iron stores estimated by grading the bone marrow stainable iron. Altogether 34 patients showed increased bone marrow iron stores and serum ferritin concentrations greater than controls; four patients showed absence of iron in the marrow, and three of these had subnormal serum ferritin concentrations. Serum ferritin assay represents the best method of repeatedly monitoring the exact amount of iron therapy needed by patients with chronic renal failure, particularly those on regular haemodialysis.