In 20 patients undergoing continuous ambulatory peritoneal dialysis (CAPD), serum ferritin concentration (radioimmunoassay) was compared with bone marrow iron stores (graded semiquantitatively). A close correlation between the two parameters was found. Patients with decreased iron stores (8/20) had serum ferritin concentrations below 70 ng/mL; those with normal or increased iron stores had concentrations above 96 ng/mL. It is concluded that serum ferritin concentrations adequately reflect bone marrow iron stores and are useful as a guide to iron-replacement therapy in patients undergoing CAPD.