Serum ferritin was measured by immunoradiometric assay, and stainable bone marrow iron was assessed semiquantitatively in 38 patients (19 8 , 19 9 ) with chronic renal failure (7 nondialysed uremic patients, 14 patients on regular peritoneal dialysis and 17 patients on regular hemodialysis). Serum ferritin was also measured in 60 healthy subjects (30 6,30 0 ) . There was a good correlation @<0.001) between serum ferritin and marrow iron in the total group of uremic patients.Patients without stainable marrow iron (n =19) had a geometric mean serum ferritin of 51 ng/d (range 6-136), mean serum ferritin in patients with "normal" marrow iron (n=14) was 326 n g / d (range 112-1 120) and in patients with slightly increased marrow iron (n =5) 634 ndml (range 480-960). Male patients had higher mean serum femtin (244 ng/ml) and higher marrow iron score than female patients (76 mg/ml) @<0.01). All patients had higher mean serum ferritin (141 ng/ml, range 6-1 120) than the healthy subjects (46 ng/ml, range 5-285) @<0.001). Patients with serum ferritin levels of 4 1 2 n g / d had no stainable marrow iron, while those with fenitin levels of >136 n g / d had adequate marrow iron stores. The serum fenitin concentration appears to be a reliable indicator of stainable marrow iron content in uremic subjects. Regular monitoring of serum ferritin may be a guide to appropriate iron supplementation and reduces the need for repeated marrow punctures in the assessment of iron status in chronic renal failure.