Serum erythropoietin (EPO) was measured by radioimmunoassay in 67 patients with rheumatoid arthritis (RA). Twenty of these patients judged to have iron deficiency anemia, based on reduced serum ferritin levels, had higher serum EPO levels than did the 24 other anemic patients with normal or elevated serum ferritin levels. A significant negative correlation between serum EPO and hemoglobin concentrations was noted in the former group, but not in the latter. Human recombinant erythropoietin (r-EPO) was administered to 6 anemic patients with RA, resulting in improvement of anemia in 4 patients, 2 of whom showed no change in RA activity. These findings suggest a suppressed serum EPO response ot anemia and the effectiveness of r-EPO in treating anemia associated with RA.