Serum immunoreactive erythropoietin (EP) levels were measured in 116 patients with rheumatoid arthritis (RA) and 20 control patients with iron deficiency anemia. Serum EP levels were significantly higher in the 46 anemic RA patients than in the 70 nonanemic RA patients (mean +/- SD 31.0 +/- 19.8 mU/ml versus 16.8 +/- 12.4 mU/ml; P less than 0.0001). Furthermore, although a significant inverse correlation between the serum EP level and the hemoglobin value was present in the anemic RA patients (r = -0.57, P less than 0.0001), the regression coefficient describing the relationship between serum EP and hemoglobin was significantly lower for the anemic RA patients than for patients with iron deficiency anemia (F = 6.01, P less than 0.025).