Marrow hemosiderin patterns in 500 patients were analyzed to determine their diagnostic value. The quantity and predominant particle size of iron was evaluated. In each type of anemia studied (iron lack, primary or secondary refractory, hemolytic, post-hemorrhagic, and vitamin B12 deficiency) excellent correlation of the hemosiderin pattern with known ferrokinetics was obtained. Confirmation of the previously reported finding of absence of marrow iron in polycythemia vera was made in all cases studied. Furthermore, a hemosiderin pattern that differs from that of cirrhosis due to other causes was observed in primary hemochromatosis. It is concluded that direct visualization of marrow hemosiderin patterns provides an accurate, rapid, and simple means of evaluating iron dynamics. In additon to offering a rational basis for the clinical study of anemias, the procedure is useful in the initial diagnostic approach to polycythemias. Results to date suggest that hemosiderin patterns may be of singular aid in the differentiation of primary hemochromatosis and cirrhosis. In contrast to ferrokinetic studies, the simplicity of marrow hemosiderin evaluation lends itself to routine application.