Serum erythropoietin levels (s-Epo) were measured by radioimmunoassay in 61 consecutive anaemic patients (Hb < 12 g/dl) with myelofibrosis with myeloid metaplasia (MMM). S-Epo was inversely correlated with Hb (r = -0.48, P < 0.0001). When observed s-Epo values were compared with predicted levels based on the relationship between s-Epo and Hb in control subjects, all but eight patients (87%) had s-Epo levels appropriate for the degree of anaemia. The observed/predicted (O/P) s-Epo ratio was significantly lower in patients with signs of active disease, and a significant inverse correlation was found between the O/P ratio and erythrokinetic measurement of the extent of erythropoiesis (r = 0.31; P = 0.02). Circulating Epo levels were appropriate for the variations in Hb during the postsplenectomy period in three patients. In conclusion, this study does not support the idea that therapy with erythropoietin should be extensively used in anaemic patients with MMM, but rather that it should be considered only in selected cases.