Erythropoietin, the major humoral regulator of red cell production, was reported to stimulate various levels of megakaryocytopoiesis. We investigated levels of thrombopoietin (TPO) and erythropoietin (EPO) in patients with reactive thrombocytosis (RT) and clonal thrombocytosis (CT). 17 patients with RT and 18 patients with CT and 15 healthy subjects were enrolled into the study. TPO levels were higher in both patient groups than the controls. EPO levels were significantly higher than these in reactive thrombocytosis and in controls (p < 0.05). We suggest that besides TPO, EPO may play an important role in the pathogenesis of RT.