Serum concentration of ferritin (Ft) and its glycosylated fraction (Ft-Gl) and intraerythrocytic ferritin (Ft-e) concentration were measured in 26 patients with anemia and active rheumatoid arthritis. Patients were divided into 2 groups according to the presence of anemia of chronic diseases (n = 13) or associated ferropenia. Unlike the first group, patients with associated ferropenia had lower concentration of the above parameters than 31 control subjects. The logarithmic value of FT (log FT) directly correlated with globular sedimentation velocity. Ft-Gl and log Ft-e correlated with transferrin saturation (r = 0.603, p less than 0.01 and r = 0.444, p less than 0.05). Log Ft-e also correlated with Ft (r = 0.504, p less than 0.01). The probability of ferropenia when Ft was 60 micrograms/l or lower was 0.91, and when Ft-e was 1.5 ag/cel or lower was 0.66. It is concluded that the ferropenic status in active rheumatoid anemia decreases the iron dependent synthesis of ferritin (Ft-Gl) more than that mediated by the acute phase response. The intraerythrocytic content is low due to the scanty iron supply to the erythroblast. Ft is more efficacious than Ft-e in the diagnosis of ferropenia.