To measure serum soluble transferrin receptor (s-TfR) levels in patients with rheumatoid arthritis (RA), sera were obtained from 50 Japanese RA patients and 20 healthy subjects. Both s-TfR and serum erythropoietin (EPO) levels were measured by enzyme-linked immunosorbent assay (ELISA). Routine laboratory tests were also performed, including peripheral blood analysis and determination of erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), rheumatoid factor (RF), serum iron levels, total iron-binding capacity (TIBC) and serum ferritin levels. The s-TfR levels in the 50 RA patients (mean +/- SD, 1,801 +/- 512 ng/ml) were significantly higher than those in the 20 control subjects (1,316 +/- 345 ng/ml). There were no differences in the values of s-TfR between men and women in either group, or between RA patients over and under 50 years old. Serum EPO levels in 47 RA patients were as low as 14.0 +/- 10.1 mlU/ml (mean +/- SD), ranging from 3.9 to 58.7 mIU/ml (normal range 2.8-17.2 mlU/ml), unrelated to low haemoglobin concentration. The s-TfR levels in RA patients showed negative correlations with red blood cell count, serum iron level and haemoglobin concentration, and positive correlations with ESR and serum EPO levels. However, there were no correlations between s-TfR level and markers of inflammation such as CRP, platelet count or RF titre. In conclusion, s-TfR level in RA patients could be a marker of erythropoiesis rather than of joint inflammation.