Serum ferritin and bone marrow haemosiderin iron was studied in 50 non-dialysis patients with chronic renal failure, and in 53 healthy subjects. S-ferritin was correlated to marrow iron both in patients with renal failure and in healthy subjects (P less than 0.001). Geometric-mean S-ferritin in patients with 0- (1+) marrow iron was 33 micrograms/l, 1+ marrow iron 166 micrograms/l, and 2+ marrow iron 519 micrograms/l. Healthy subjects with 0- (1+) marrow iron had a mean S-ferritin of 16 micrograms/l and those with 1+ marrow iron a value of 65 micrograms/l. S-ferritin levels were higher in patients than in healthy subjects at all marrow iron grades (P less than 0.001). Healthy subjects with S-ferritin less than 15 micrograms/l had absent or reduced marrow iron, while those with S-ferritin greater than 30 micrograms/l had normal marrow iron. Using a critical S-ferritin value of less than or equal to 20 micrograms/l, the diagnostic efficiency in terms of diagnosing absent or reduced marrow iron was 0.90 (PV pos = 0.85, Pv neg = 0.91). In patients with renal failure S-ferritin less than 60 micrograms/l indicated absent or reduced marrow iron, while values greater than 80 micrograms/l were associated with normal marrow iron. The diagnostic efficiency of S-ferritin using a critical value of less than or equal to 60 micrograms/l was 0.94 (PV pos = 0.93, PV neg = 0.97). S-ferritin is a useful indicator of marrow iron stores in patients with chronic renal failure.