Serum ferritin was correlated with bone marrow iron stores in 35 elderly anaemic patients. All 19 patients with low serum ferritin had iron deficiency as assessed on bone marrow examination, whereas six patients with low or absent bone marrow iron stores had serum ferritin within normal range. A low serum ferritin invariably indicates iron deficiency and has a better correlation than low serum iron. Serum ferritin should be included in the initial investigation of the anaemic elderly.