The hematologic status of 265 patients with rheumatoid arthritis was assessed. In the group as a whole, a mild depression in the hemoglobin concentration and mean cell volume (MCV) was associated with an increase in the red blood cell distribution width (RDW), erythrocyte sedimentation rate (ESR), and platelet count. Bone marrow trephine biopsies and further measurements of iron status and disease activity were done in [a further] 38 more anemic patients, and the findings in those with absent marrow iron (iron deficiency) were compared with those having stainable stores (anemia of chronic disorders). The RDW was raised in both, and there was no significant difference between the two groups. The concentrations of nonheme iron in the marrow and of serum ferritin were significantly lower in the iron-deficient group, but the geometric mean serum ferritin of 34 micrograms/L was still a good deal higher than that associated with uncomplicated iron deficiency. This was presumably because of the fact that the serum ferritin, which was significantly correlated with the ESR (r 0.55; P less than 0.0004) and C-reactive protein (CRP) r 0.41; P less than 0.01), was also functioning as an acute phase protein. While there was a weak correlation (r 0.37; P less than 0.04) between the marrow nonheme iron and the serum ferritin concentrations, it disappeared when nonactive patients with normal CRP concentrations were excluded. The absence of a correlation is unlike the findings that have previously been noted in other chronic inflammatory conditions and in neoplasia. This raises the possibility that serum ferritin concentrations in rheumatoid arthritis may reflect, in part at least, another store of iron located in affected joints.