Erythrocytosis is not a feature of essential thrombocythemia (ET); this is the most important difference between ET and polycythemia vera (PV). Transformation of ET to PV has only rarely been described. We have reviewed the blood cell counts of 170 ET patients with a median follow-up of 63 months (range 11–313). Eleven of 170 patients (6.5%) developed erythrocytosis at a median of 29 months (range 12–138) after the diagnosis of ET. According to the present results, the development of erythrocytosis in patients with ET is not a rare phenomenon.