Thirty-five anemic patients with rheumatoid arthritis were studied to determine the relationship between serum ferritin levels and body iron status, as assessed by the grading of bone marrow iron stores. The incidence of greatly reduced or absent marrow iron stores was 60%. Peripheral blood smear, RBC indices, serum iron, and iron binding capacity correlated poorly with stainable marrow iron. Serum ferritin levels only correlated approximately with iron stores, and in iron deficient rheumatoid patients the levels were higher than would be expected in patients with uncomplicated iron deficiency. The study shows that reduced marrow iron stores is common in patients with rheumatoid arthritis, and that the serum ferritin concentration may provide a useful indication of reduced body iron stores in these subjects, but only if a range of normal values can be established for this disease.