The amount and effectiveness of erythropoiesis was measured using 59Fe in 10 patients with the anaemia of chronic disease and in 10 iron deficient patients with a comparable degree of anaemia. In both conditions the anaemia was the result of the failure of the marrow to compensate for a modest degree of peripheral haemolysis but ineffective erythropoiesis was significantly greater in iron deficiency than in chronic disease. The results suggest that although the peripheral blood picture is similar in both conditions the anaemia of chronic disease cannot be attributed simply to iron deficient erythropoiesis.