Serum concentration of thrombopoietin (Tpo) and interleukin 6 (IL-6) were measured in 37 patients with thrombocythaemia and in the healthy volunteers. 27 patient were classified as persons with chronic myeloproliferative disorder (MPD) and 10 as with reactive thrombocythaemia (RT). The mean plasma Tpo concentration did not differ statistically between the groups. IL-6 levels were significantly higher in patients with RT than in patients with MPD and controls. The mean plasma IL-6 levels was lowest in patients with essential thrombocythaemia. In conclusion, serum IL-6 concentration may be useful in the differential diagnosis of thrombocytosis.