Serum erythropoietin (s-Epo) was measured with a sensitive radioimmunoassay method in 58 patients with classical rheumatoid arthritis (n = 41) or seronegative spondyloarthropathies (n = 17). Epo was significantly (P less than 0.001) increased and on an average two times higher than in a healthy population. A correlation was found between Hb and s-Epo (r = -0.46, P less than 0.005), indicating that these patients respond to anaemia with an increase in s-Epo. In order to investigate if inflammation has a direct influence on s-Epo levels a short period of corticosteroid treatment was given to rapidly decrease inflammatory activity. No increase in s-Epo was seen after 1 week. Furthermore, there was a correlation between s-Epo and ESR in all patients (r = 0.59, P less than 0.01). These results indicate that s-Epo is directed by the Hb level, which in turn is influenced by the inflammatory activity: a higher inflammatory activity gives a lower Hb and an increase in s-Epo. In comparison to previously published figures for the relation between Hb and s-Epo these patients seem to have an ordinary Epo response. We conclude that the anaemia of patients with chronic inflammatory joint disease is not caused by a diminished Epo production.